We don't know much about Misa's upbringing, though it seems that she was close to her parents. They were killed in a burglary before Misa's eyes when she was nineteen, and their death affected her deeply. A year after their death, she begins her modeling career with a spread in the popular magazine Eighteen. She begins to gain popularity almost immediately, but things change drastically for her a short time later, when two things happen. The first of these is that the mysterious person known only as 'Kira' writes the name of the burglar responsible for killing her parents into his Death Note. I'm sure Misa must have heard of Kira before this, because of the many, many news reports that have focused on him, but it's this event that changes her view of him from a presumably neutral view to complete devotion. She sees him as her personal savior, something which is only reinforced a few weeks later.
The second occurrence, about six weeks later, is an even more dramatic one for Misa. She is attacked by a stalker who is intent on killing her. The Shinigami Gelus, who has been watching her and has fallen in love with her, witnesses the attack. Gelus writes her attacker's name in his Death Note, killing him. However, because this act - falling in love with a human and using the Death Note to protect them - is forbidden by the rules of the Shinigami, Gelus dies almost immediately after writing the name, fading into dust before Misa's eyes. A second Shinigami, Rem, witnesses the entire thing, and several days later, comes down to Earth herself, having decided that Misa should take ownership of Gelus' Death Note. After meeting Rem and having the details explained to her, including how to kill a Shinigami, Misa decides to both begin using the Note herself and to make the trade for the Shinigami Eyes. In exchange for half of her lifespan, she can now see the true names and lifespans of any human around her.
This is the start of her activities as the second Kira. Misa goes to Tokyo, and prepares a series of videotapes along with a letter. She sends all of these to the TV station Sakura TV, which is known for its sensationalism in reporting. The letter states that its writer is Kira, and that the first videotape will prove this to be true. If the proof on the first videotape is enough to convince them, they are to air the second, third, and fourth videotapes at specific times. It goes on to state that the tapes will send a message from Kira to the world. Whether it's due to greed or actual belief isn't unclear, but the director of Sakura TV, a man named Demegawa, makes the decision to air the second tape at the specified time.
They begin by reporting the contents of the first tape, which states the names and times of death of two men, both of whom were only arrested a few days prior; both of them died at the exact times specified, information that only Kira would know. Sakura TV proceeds to air the second tape as specified, it contains a frightening message - to turn to another station. The anchor of this station, who had often called Kira evil in his news reports, dies on the air. The tape goes on to say that one demonstration is not enough, and that a second commentator, who had also spoken out against Kira, would also be killed. This happens just as specified, while L and the members of the Japanese National Police Association's task force watch from their headquarters, in stunned silence. (L is considered the world's greatest detective, and has contacted the NPA after becoming interested in the case. The task force itself is small, made up of a small number of officers, headed by Yagami Soichiro, who are focused on solving the Kira murders.) After the two deaths, the message begins: Kira hates evil and loves justice, and does not want to kill innocent people. In addition, Kira sees the police as allies, not enemies, and hopes for their cooperation. It goes on to state that if no one tries to catch Kira, no innocent people will die - and that even if people don't support Kira, as long as they don't broadcast those thoughts in public, they will still be safe. Kira goes on to state that the world will soon be a much better place, and that everyone will see that and agree. The tape ends with a question to the police: are they ready to cooperate with Kira in creating an ideal, just world? They have four days to decide this, and Sakura TV will air their answer at that time. There are two videos prepared, and the station will air them accordingly: one if the police say yes, and a second if they say no.
While the tape is airing, Ukita of the NPA task force has decided to go to Sakura TV in time to stop it. The other networks begin to broadcast from outside the station - with their anchors staying off-camera for their safety - and it's just in time to see him falling to the ground, dead. Yagami Soichiro sees this from his hospital bed - he had been hospitalized for a heart attack brought on by overwork - and decides to take action. He takes an armored police vehicle and storms Sakura TV, eventually getting the original copies of the tapes and letters, along with copies that Demegawa has made. After this, the task force meets, to decide their plan. They decide to announce that they won't be cooperating with Kira, and at the same time, will invite Yagami Light - the true Kira, a fact unbeknownst to anyone else - to join the investigation team. They show him the original, unaired video, and Light is forced to admit in front of L and the NPA that there may be a second Kira. At this point, the task force asks Light to help them in the investigation by making a new video to send to Sakura TV - one that will purportedly be from the real Kira. Light agrees, and the new video - stating that the one that originally appeared on Sakura TV is a false Kira, offering to show leniency to the false one because they are acting to support Kira's ultimate goal, and finally, to please stop targeting members of the police force - airs that very evening.
Misa is delighted - Kira has responded to her! - and makes a reply video of her own. The video states that she wants to meet Kira, and that the real one can prove that he is such by revealing his Shinigami to her when they meet. She also says, "I don't think you have the eyes, but don't worry. I won't kill you," and asks the real Kira to find a way that they can meet that won't arouse police suspicion. When there is no response, a short time later, Misa mails a video and a diary to Sakura TV, asking that the diary be broadcast. The diary is written in such a way that only those who have access to a Death Note would understand it, and particularly so in two sentences: first, on May 22, "We showed off our notebooks in Aoyama", and second, on May 30, "We confirmed our Shinigami at the Giants game at Tokyo Dome." While the task force members and L focus on the second statement, at least initially, and begin making plans to secure the area around the Tokyo Dome on that day, Light catches the meaning behind the first immediately. The final decisions are that every place in the diary will be checked thoroughly, including Aoyama.
On May 22, Misa disguises herself in a schoolgirl uniform and glasses, and watches passers-by from a cafe in Aoyama. At the same time, Light and another member of the task force go there, keeping an eye out for anyone that would be suspicious. Misa's Shinigami Eyes help her spot Light and pinpoint him as Kira immediately; however, she doesn't immediately speak to him. Instead, with that knowledge in hand, she leaves and begins to research everything she can about Light. She finds his full life history after only a bit of searching - where he went to school, where he goes to university, and most importantly, his home address. Next, she records a new video and sends it to Sakura TV, thanking the police and television stations for their cooperation and stating that she has 'found Kira'. This video airs on May 25, and when it does, L immediately has a video response prepared, asking the second Kira not to meet the first and to come clean about what they know. At the same time this message is airing, Misa is on her way to the Yagami house, to 'announce herself to Kira'. She walks straight up to the front door and rings the bell, introducing herself and saying that Light left an important notebook at school. Light immediately recognizes it as a Death Note, touches the corner so that he can see Rem, and rushes Misa up to his room.
At this point, Light knows that Misa is the second Kira, and that she hasn't had time to make any sort of contact with the police. His first question to her is "how do you know I'm the real Kira?" She explains that she's made the trade for the Shinigami Eyes, and that the only people who don't have life spans displayed are Death Note users - a fact that seems to surprise Ryuk. Rem steps in, confirming that what Misa said is the truth, and adding that she warned against Misa meeting 'Kira' and telling him her real name, but that the girl insisted on doing so. Light chides her, saying that if she had been caught, it would have been disastrous. The girl agrees, but says that she hasn't been caught, and if she does as he says from now on, she won't be. Misa goes on to say that she will be Light's eyes, and find L's real name for him, if he will make her his girlfriend.
His initial reaction is to say that this is impossible, because there was heavy police presence in Aoyama the day she claims to have found him, and anyone who was there was certainly caught on camera. If two people seen on camera were then later seen together, that would be a clue for the police. Misa laughs this off, explaining that she was disguised, and shows Light a picture. He agrees that she isn't recognizable in the picture, but then questions the videos she sent to Sakura TV, asking about the fingerprints. She explains that her prints aren't anywhere to be found on the envelopes. Until recently, Misa explains, she had a friend that was heavily into the occult. She made some fake ghost footage and showed it to her friend, convincing her that it needed to be sent to television stations. Her friend then dubs the footage onto 10 tapes and prepares the envelopes. Misa went on to record over that false footage, adding sound, and being careful not to leave her own prints. Light asks where this friend is, and Misa says, simply, "If you want me to kill her, I'll do it right now."
She goes on to add that if Light can't trust her, he can take her Death Note. As long as Light is just holding it, Misa will still be its official holder, a fact she confirms with Rem. If Light accepts it, Misa can't kill people, there is only one person evidence can be recovered from, and furthermore, she can be killed if she ever becomes a burden. Light scoffs, saying that she might have removed pages, but Misa objects, saying that she's never thought of doing such a thing, and that Light could easily tell if there were any missing pages. She goes on to ask if the notebook's only purpose is killing people, and Light confirms it in detail. At this point, Misa breaks down, stating that she doesn't care if Light just uses her. She explains that her parents were killed by a burglar in front of her. The trial was delayed a number of times, until Kira punished the burglar, and because of that, she views him as her personal savior.
The next question is a harsh one: how killing innocent police officers was any different than the burglar that killed her parents. Misa protests again, stating that to defeat evil, there had to be sacrifices - and more than that, it was the only way she could think of to get Kira to notice her. She wanted to thank him, and finally, to see him. At this point, Light says that he can't be her boyfriend, but he can play the part. This is enough for Misa - she thanks Light, and promises she'll do her best to try and make him love her, too. The two go on to discuss several subjects in detail: how to kill a Shinigami, how the Shinigami Eyes work, how Misa was mailing the tapes (she took trains to different cities in order to mail them), and what to do about the rest of the remaining tapes. They decide to send one final tape from the second Kira and destroy the remaining tapes and envelopes. They then discuss what to do about L and the task force. Light explains that he's already under some suspicion from L, but it's because of that fact that the two of them have been able to work together; Light is working with L in order to gain his trust. As such, if Light seems to have a new friend who he's spending a lot of time with, and there's a change in the way both Kiras are acting, that would throw suspicion on both Light and Misa. Because of this, Light says he'll have to be seen with other girls. Misa freaks out at this point, saying that she can't stand for him seeing other girls, because it's different than simply being used. Light threatens her at this point, saying that now he has both notebooks and can easily kill her. Rem steps in, saying that she won't allow that - if Light kills Misa, or is about to, she'll kill him, even at the loss of her own life. This ends their first meeting.
Misa proceeds to try and spend as much time with Light as she can, beginning by visiting him at home just two days after their first meeting. It's Rem that Light's more interested in at first - she reveals that she's been watching Misa for some time, and has developed some feelings for her, which is why she's willing to kill herself if Light tries to hurt Misa. Light is able to spin this to his advantage at this point, asking Rem if that means Misa's happiness is also hers. The Shinigami agrees, and then Light asks Misa if his happiness would also mean hers. She enthusiastically agrees to this. He then asks her for a favor - asking Rem to kill L. He explains that if either of them are caught by L, then they could never be happy. Rem thinks it over briefly, and decides that even though she doesn't like Light, since she has no attachment to L, that she can do this. Light asks her to wait until his command before doing so. After this, Light asks for Misa's cell phone number so that they can stay in contact with each other. Misa gives hers, then asks for his in return. Light refuses, saying that his might be bugged, but Misa has a solution - she has extra phones, so she gives him one of those. She promises to call and email him every day, but he refuses, saying he will keep the phone turned off until he needs to contact her - and that now she needs to leave. Misa is disappointed by both of these occurrences - first, Light's not going to call her very often, and second, that she needs to leave. Light puts an end to the conversation by kissing her, and after that, Misa leaves content and happy.
She doesn't stay away from Light for long, though. Using the excuse that she has a photo shoot near the university Light attends, Misa surprises him by showing up on campus unexpectedly the next day - and while he's speaking with L, no less. While it surprises him, it doesn't take long for Light to realize the situation is to his advantage. Misa can see real names and lifespans, after all, and if he can get L's real name from Misa, it saves him the trouble of having Rem kill him. When L introduces himself by the false name 'Ryuga Hideki', it throws her off momentarily, because it isn't the one she sees, but she quickly turns on the sparkle, especially when L claims to be a big fan of hers. The three of them draw a crowd, at that point - 'Misa-Misa' has a lot of fans that gather around her, as soon as L draws attention to her. Someone in the crowd tries to grope her not long after, and she yells loudly. (In reality, the 'grope' is L taking one of Misa's cell phones; while she's distracted, he's able to take a second out of her bag.) Misa's manager pulls her away at that point, telling her she'll be late for her photo shoot if they don't hurry. The group breaks apart and says their goodbyes.
Once Light thinks L is out of earshot, he tries calling one of Misa's cell phones. Unexpectedly, L turns around and is holding the phone. He claims that someone in the crowd dropped it, but Light recognizes it. He plays it off and is just about to call another of her numbers, but then L's phone rings. The news is bad for both Light and Misa - she has just been arrested on suspicion of being the second Kira, because fibers collected from her apartment match ones found on the adhesives used to seal the envelopes containing the tapes that the second Kira has sent out.
Misa is kept in confinement for some time, and it's towards the beginning of this confinement that she loses consciousness and wakes up at Landel's.