PERSONALITY
As it should be clear from her past, Rika has seen her share of misfortunes. Even though she's only twelve, she has the maturity of someone well beyond her physical years. However, she knows it would alarm her friends if she suddenly became more mature, so tends to put on a front. To her friends, Rika is incredibly cheerful and kind, the type to make compromises instead of letting a conflict arise, unless said conflict is just playful. She is very "cute" and naive in the eyes of her friends, and certainly not someone you'd talk to about "adult matters", like why that movie is rated R. Rika is also known for her cute phrases ("Nii" and "Nipa" in particular, which are both smiling sound effects) and for using sound effects in her speech (The storm went "Boom boom" and "Flash flash" nano desu!) as well as ending some sentences in "nano desu" (That how it is!).
Even with her front, Rika is mature, as is expected from someone who lives alone.
Deep down, Rika isn't the cheerful happy girl she appears to be. At least, not all of the time. She's a cross between an optimist and a pessimist, unsure of whether to have faith in a happy future, but willing to keep trying until she can't anymore. Sometimes she'll fall into "emo slumps" where she sits around and drinks wine (which, by the way, she loves). However, she often gets out of these slumps pretty quickly,and goes back to trying her very hardest to find a happy ending.
Since this RP requires that her memories be wiped, Rika is not likely to be quite as emo. She is more likely to use the front personality since she cannot remember anything to be said about, but her "dark side" ("Frederica") remains, as it is a core part of her. Luckily, she will not have any reason to emo out until she begins to remember, and will mostly be her cute and cheerful self.
HISTORY
Rika is the one character in the series that keeps her memories in every world. She's been to all of the worlds.
Since you said spoilers are okay, I'll start with some things that will make this much less confusing to explain xD
In Higurashi, there's a parasite that affects a person's brain called the "Hinamizawa Syndrome", simply because the parasite can only survive in the geographical conditions of Hinamizawa. This parasite affects every villager, but doesn't harm them until they leave the village. At that point, when it leaves Hinamizawa, the parasite becomes active. It starts by making the person infected paranoid, so much so that they often end up becoming homicidal, and then it causes them to become "itchy". They itch themselves (usually their throat or wrist) and become convinced they have bugs under their skin. When they start to bleed, their paranoia causes them to see the bugs and itch more to get them all out. In the end, at this final stage of paranoia, they scratch an artery and bleed to death.
There is no cure, but the Irie Clinic is researching a medicine that halts the progress and keeps you from being paranoid and homicidal (as long as you take your meds). In order to use this, you need to blood of the "Queen"; the queen is the only one in Hinamizawa who is not driven crazy by the parasite. But when she dies, everyone infected, regardless of whether or not they left Hinamizawa, goes crazy within 48 hours. The Furude head of house is the Queen, and the latest in the bloodline is Rika, so her blood is needed. Houjou Satoko is very infected (at the "final level" before suicide), so takes the medicine every day (but she thinks they're vitamin injections). This gives Rika easy access to the 'cure', and because she's so heavily involved, she knows everything there is to know about the parasites. No one else but Dr. Irie and Nurse Takano know about it yet.
But here are the universes you visit in the anime...
Onikakushi-hen is the first story arc. In this arc, Maebara Keiichi leaves the village for a funeral.When he comes back, he learns of a case five years ago in Hinamizawa where a construction worker building a dam (that would put the entire village under water) was brutally murdered and split into five parts. "They still haven't found one arm, have they?" is the quote from a free lance photographer, Tomitake, that inspires Keiichi's paranoia. He becomes convinced that his friends from school, Ryuuguu Rena and Sonozaki Mion, are behind the killings. This arc shows you everything through Keiichi's eyes, so they appear scary, but should you listen to the dialog, you'll find it's all friendly and concerned dialog that Keiichi is mishearing. In the end, he kills Rena and Mion, then scratches out his throat and dies in a telephone booth.
In Watanagashi-hen, you meet Mion's twin sister Shion. She's very sweet and kind to Keiichi, but in the answer arc, Meakashi-hen,you learn that she's flirting on purpose to try and get Mion (who actually has a crush on him) to take action in winning Keiichi's heart when she actually has 'competition'. Watanagashi-hen involves Shion and Keiichi sneaking into Oyashiro-sama's shrine with Takano (a nurse at Irie Clinic) and Tomitake. They become "cursed" for what they did. Takano and Tomitake die on Watanagashi due to the yearly curse while Shion and Keiichi are haunted. Anyone they tell about what they did is killed and they believe it is leading up to them. Near the end of the arc, Keiichi realizes that the "Shion" he is talking to is behind the murders because of her wording of sentences, and confronts her with Rena (Rika and Satoko have become victims at this point). They think Shion is Mion and Shion eventually knocks Keiichi out, then after he recovers from that, shows up and stabs him.
However, in the answer arc, it's revealed that it was Shion committing the murders, and that she was using her twin's identity. An important Rika moment in this arc is that Rika recognizes Shion for having the Hinamizawa Syndrome. She takes some of the medicine for it and tries to inject it into Shion, but Shion is so paranoid that she thinks Rika is trying to poison her. Rika is the Queen, and essentially is the parasite, so when Shion injects the medicine into Rika instead, it had a very negative effect on her. Since Rika has clearly done this in other worlds, she knows that Shion intend to torture her before she dies, so commits suicide.
Shion did all this for love, in a twisted way. The boy she loved, Houjou Satoshi, was supposedly "demoned away", and she thinks the house heads in Hinamizawa are the ones behind the mysterious deaths. So her original plan is to kill everyone involved in Satoshi's death. She 'wins' and then falls off the balcony of her apartment and dies.
Tatarigoroshi-hen involves child abuse (it's probably the most depressing too). Houjou Satoko's parents died a while ago, so she's been living with her uncle and aunt. But after her aunt was killed by the curse, her uncle rarely comes home. In this chapter, he comes back, so Satoko has to go live with him again. He severely abuses her, mentally and physically, and has her convinced that she can't tell anyone or get help because this is a "test" to make up for all the trouble she caused her older brother,Satoshi. If she doesn't do what he asks, he's threatened to burn down Satoshi's bedroom, and to Satoko, that will be the final sign that her brother can never come home. It is Rika that explains the situation. Keiichi is the lucky infected one in this arc, and goes out to kill Satoko's uncle. He ends up dragging Satoko around for a while, and they learn that Dr.Irie has died from an overdose on sleeping pills. They go to the shrine, because Satoko needs new clothes and they're at Rika's house, and Keiichi finds Rika, dead and disemboweled at the foot of the shrine. Satoko comes back, assumes Keiichi did it (then again, the genius boy did pick up a bloody ax to swing at some crows after Rika's...er...I'll shut up) and shoves him off a bridge. He wakes up after being KOed and learns that there was a "volcanic gas" eruption in Hinamizawa and that every villager was killed. The arc ends here.
Himatsubushi-hen is a prequel, and happens five years before the start of the other arcs, which is also before the curse begins. Akasaka Mamoru comes to Hinamizawa to investigate a kidnapping and meets younger Rika she's about 6 or 7 at this point). He learns of the dam project, which would put Hinamizawa under water, and how the villagers are protesting. "Dark Rika" first appears in this arc, warning Akasaka to go back to Tokyo. She also tells him that the dam construction will be stopped and that, in five years, on June 19, she'll be murdered. He doesn't believe any of this. Later in the arc, she cuts all of the telephone wires so he can't call home, saying that if he did call home, he'd be very sad. When he gets back to Tokyo, he learns that his wife has died, on the exact night that Rika cut the wires, and before she warned him to go home. Five years later, all of the predictions Rika made are confirmed, and Akasaka feels very guilty.
Tsumihoroboshi-hen is next. In this arc, Rena becomes slightly aware of the syndrome, but she thinks it is because of aliens. Naturally, no one wants to believe her. She realizes she has the syndrome and will die soon, so holds the school hostage and demands that the police find a cure. In the version of this arc you see, Keiichi remembers the first arc where he went crazy and becomes determined to help Rena before she makes the same mistakes (Rika forgives Keiichi for killing everyone as well). Rena realizes she's hurting her friends and stops due to Keiichi's efforts,and Rika realizes (slightly) that fate can be changed if you work together; In the manga-only arc, Yoigoshi-hen, Rena burns down the school and kills everyone in it. Either way, the Hinamizawa Disaster comes, poisonous gas goes everywhere,and everyone dies anyways, regardless of whether or not Rena burns the school down.
There is also the anime-only arc, Yakusamashi-hen, where Satoko catches on to the fact something is wrong and tries to save Rika. When the night that she will be killed comes, Rika has Satoko hide and allows herself to be taken without protesting. This sacrifice suggests that, in past worlds, Satoko has possibly protested Rika's being kidnapped and been killed to be silenced. Satoko does some running around, gets knocked out, wakes up and finds the village after the disaster. She freaks out and goes into mental shock, so cannot talk or interact,just sits in the hospital. Before the police can get answers out of her, one of the nurses puts bubbles in her IV and she dies when the bubbles enter her bloodstream.
My Rika's final arc will be Minagoroshi-hen. In this arc you meet Hanyuu, the spirit that accompanies Rika and creates the new worlds when she dies. Keiichi teaches Rika that fate can be changed, and Rika realizes that all her evidence so far points to the Nurse,Takano, as the murderer. The six friends fight fate to try and avoid Rika being murdered, but on their way, Keiichi is shot. Shortly afterwards, Mion sacrifices herself as a decoy,followed by Rena, before Shion, Satoko and Rika are caught. Takano shoots Shion and Satoko in the head. Rika says she wants to be awake when murdered so she can burn Takano's face into her memory and do something about this in the next world.