This contains spoilers for all of .hack//G.U. vol. 1 and part of volume 2. It also contains some spoilers for the original games, as well as the Terminal Disk. Keep this in mind.
Also: Rewatching cutscenes, tracking down the Terminal Disk, and reading game scripts takes a long time. As such, I'd appreciate others not taking my app. ♥
Endrance's history begins seven years before the events of the G.U. Games. At age 13, his player, Kaoru Ichinose, began playing The World: R:1, creating a Wavemaster character named Elk. He began playing because he was quite shy, and was hoping to make friends. He made fast friends with a cat player named Mia; the two of them became inseparable. It is Mia that introduces Elk to Kite, the protagonist of the original .hack games. Elk eventually joins Kite's team, fighting the Cursed Wave. However, his dear friend Mia begins to act strange after a time. She is eventually revealed to be an AI, hiding the 6th form of the Wave, Macha, known as 'The Temptress'. Over Elk's protests, Mia turns into Macha, and Kite fights her. After the fight, Mia changes into her original form briefly. Elk tries to heal her, but is unsuccessful. Before she fades into data, Mia thanks Elk for his friendship.
The two are briefly seen again at the conclusion of the original games, Mia having been restored to her cat form again. It is after this that at some, undeclared, point in time, Kaoru stops playing The World. (The World stops service in summer 2015, after a fire at the CC Corp (CC Corporation; the company that runs The World) headquarters destroys most of the servers running the game, as well as 80% of the game's data. CC Corporation quickly moves to relaunch the game, combining the remaining data from The World with a game in development called 'King Crimson'. During this process, Mia is destroyed, and Macha is separated from her, in what was referred to as the 'RA Plan'. This plan is unsuccessful, but as we see later, Macha's data reappears.)
He attended and graduated from high school, but he had a difficult time while in school. He had never truly overcome his shyness, and the loss of Mia - who he considered his one true friend - affected him deeply. After his graduation, Kaoru shuts himself in his room, having decided that the real world is too much for him to handle. His mother takes care of him, as he only rarely leaves.
When the new game - The World: R:2 - is launched in December 2015, Kaoru, now age 18, decides to begin playing again, creating a new character. This new character is the Blade Brandier, Endrance. Upon the creation of this character, the data from Macha attaches itself to the character, making Endrance one of eight Epitaph Users, who are capable of summoning an Avatar in the game. (The others are Haseo, Atoli, Kuhn, Pi, Yata, Sakubo, and Ovan.) An Avatar is an additional piece of character data - an 'extra form', as it were - that allows the Epitaph User to use powerful attacks and Data Drain their opponents.
It is never actually stated in game when this happens, but at some point several months after he begins playing, he is infected by an Artificial Intelligence Data Anomaly - known by the abbreviation AIDA. The AIDA takes the form of a small cat, who Endrance 'recognizes' as Mia, his dear friend from The World: R:1. After this, the cat is always seen on his shoulder, and he refers to her as 'she', claiming that 'she' is his only friend. He doesn't form parties with other players, and while others are intrigued by him, he largely ignores their presence, preferring to fight and explore the game by himself. He begins spending more and more time in The World, eventually logging out only to take care of very basic needs and to sleep.
Endrance begins fighting in the game's Demon Palace Arena after he is infected by AIDA, and it is during one of the early fights that he awakens to the power of his Avatar, Macha. He begins to use Macha in arena matches, and quickly rises through the arena ranks, eventually defeating the current emperor, Alkaid. Others challenge him after he is crowned Arena Emperor, but none of the teams were able to defeat him, due to the illegal use of Macha's power. He gains fans along the way, including a little girl called Saku - who is devoted to him - but he doesn't get close to any of them.
Haseo, the game's protagonist, encounters Endrance for the first time after he has been established as Demon Palace Emperor. He has appeared, surrounded by a black cloud of AIDA, in the Lost Ground known as Indieglut Lugh. Haseo recognizes that something is amiss, wondering what the black dots surrounding him are, but Endrance largely ignores Haseo's presence. He leaves after a few moments.
The next time we see Endrance, it's when Haseo has come to the arena to watch a match. Endrance summons Avatar Macha during the battle, but no one else that Haseo speaks to is able to see it. Haseo realizes that Endrance is an Epitaph User, and forms a team to challenge Endrance. He, Atoli, and Silabus defeat several opponents, and make it to the title match.
During this match, Endrance taunts Team Haseo, saying that 'she' is easily bored by a 'normal battle such as this', and that what the team is doing is 'Not nearly enough; at this rate 'she will never be satisfied'. Haseo fights back, and eventually, the match becomes an Avatar Battle. Haseo summons his Avatar, Skeith, after Endrance summons Macha. This surprises Endrance, but he is determined to win the match.
While summoning, however, he becomes disoriented, asking “Where am I? What is this place?” and saying that he doesn't like it there, because 'she' isn't there. He goes on to question why 'she' isn't there, asking, “Is it because I'm weak? Is that the reason why?” Haseo asks what Endrance is talking about, but he ignores the question, going on to say, “I will get stronger. You'll see. Then will you stay with me forever?”
As the fight continues, Endrance explains his earlier questions. “With you (Mia, the cat AI from The World: R:1) gone, I was all alone again. Until 'she' appeared to me. I was so happy then. Because you were my only friend, because you were all I wanted. But it's fine now. 'She' has returned. 'She' has returned from the outside world. I am no longer alone. 'She' may look a bit different now, but I knew exactly who it was right away.” Haseo asks if by 'she', Endrance means AIDA, but he says, “ 'She' is simply...'she'.” Haseo questions how he can accept something he doesn't know anything about, but Endrance doesn't reply immediately. Instead, he continues to fight.
After a few more moments, Endrance seems sad, saying that “If I don't finish this soon, 'she' will disappear. 'She' will leave me.” Haseo asks what this means, but again, the Emperor avoids the question, saying that 'she' likes to talk with people, and wondering what words Haseo will leave behind, and what feelings. Shortly after this, Macha tries to Data Drain Skeith, but is unsuccessful. Skeith, a few minutes later, does successfully Data Drain Macha.
Endrance collapses, calling to 'Mia' as he falls to the arena floor, but the cat AIDA hisses at him before disappearing into a cloud of black specks, which fade away afterwards. It's revealed in the following cutscenes that Endrance has known it was AIDA for some time, but still treasured 'Mia' despite this. Haseo wonders at this, before leaving the Arena to be crowned as Emperor himself.
The loss of 'Mia' for a second time affects Endrance deeply. He is seen wandering the streets of the city of Mac Anu for a time, looking lost and searching for 'her', before he finally giving up the search. It is at this point that he creates a 'prison' for himself, sealing himself in a block of ice under the lake at Indieglut Lugh. Saku comes to visit him regularly, but he ignores her presence, instead mourning the loss of his friend for the second time.
While this is happening, Haseo and the other members of Team G.U. (Pi, Kuhn, Yata, and now Atoli) continue to investigate AIDA occurrences in The World. It is at this point that they learn a second Arena Emperor, Sirius, has become infected. Atoli, Haseo, and the former Emperor Alkaid form a new Team Haseo, fighting through the Holy Palace. However, their efforts are cut short when an AIDA-infected player named Bordeaux PK's (Player Kills) Alkaid. This sends her into a coma in the real world. Alkaid's last words to Haseo before losing consciousness are for him to find Endrance and get him to help.
Haseo comes to Indieglut Lugh to speak with Endrance and convince him to fight in the arena again. Saku is there when he arrives. The girl accuses him of 'trying to steal Master En from her', and becomes increasingly more upset. This triggers the awakening of her own Avatar, Gorre. Haseo summons Skeith and the two fight. After the fight is over, Haseo is finally able to speak with Endrance directly.
Endrance asks Haseo to leave at first, but Haseo is insistent. Endrance goes on to explain that he has, “lost everything...I no longer have a reason to awaken in the morning.” Haseo says he isn't buying it, and that Alkaid has nominated Endrance. His reply is that none of that has anything to do with him anymore, since 'she' is gone; he says that he was only able to fight because 'she' was with him, and that it was through 'her' strength that he had any affect on The World at all.
Haseo is incredulous, asking if he was talking about the AIDA-cat. Endrance replies, “Mia...no matter what happened in our lives, she was always my friend. 'She' went away once, but 'she' came back. 'She' was all I had in this world, and that was why I did everything that I could for 'her'. Everything. Feeling joy and love...and pain at 'her' words...made me so happy.”
Haseo says, “That so-called cat you're talking about is named AIDA. And she's a mysterious, human-eating monster.” Endrance denies this, and adds, “ 'She' was the only one who ever needed me! But now 'she' is gone. And there's no one else who needs me. AIDA, or whatever, has nothing to do with me. All I want is to be left alone to think about 'her'. ” The response: “You've just been denying anything that wasn't her. You haven't done anything, and you're giving up without a fight!”
Endrance insists again that no one else has ever needed him, and it's at this point that Haseo finally says, “I need your help”. It takes a bit more convincing, but Endrance does eventually come out of his ice prison, and joins Atoli and Haseo at the arena in time for their next match.
This point - the point at which he comes out of the ice after Haseo speaks to him - is the point at which Endrance will wake in Landel's.