Take aspects of Persona 3 (the main inspiration for this AU), Kingdom Hearts, FFVIII (LOL if you see why), and the team-of-superheroes genre and and put them into a blender -- you get out something like the gigantic backstory of this Mytho, alter persona "Prince Siegfried". If I have anything to change about this bio, I will, so changes may occur without warning.
The journal name "schüchtern prinz" means in German "shy prince", referring to the more introverted, quiet persona of Mytho contrasting the bold, noble Prince Siegfried.
TL;DR version: Mild mannered Mytho (yes, with his heart!) transforms into the classy badass Prince Siegfried alongside his buddies, fighting Shadows together to keep those dark beings from stealing people's hearts!
Biglong backstory version: Mytho lived in an orphanage in the stately old town of Goldkrone for several years, with little to no memory of his childhood before having been taken there, except having been found wandering out alone in the street as a young adolescent. While he had no name to remember or none from where he was found, his pale hair, fair complexion and innocent nature gave the matrons an impression of a boy from a fairy tale, thus naming him Mytho after the word "mythos", the word myth in German. The years he spent at the orphanage made him somewhat timid and submissive, as he was so kind and gentle that some of the more dominant children often picked on him or took advantage of his at times naive nature.
At his first coming to the orphanage, there was one young child who almost immediately took it upon himself to be Mytho's protector from the rougher children -- a boy named Fakir, whose parents had died before he was sent there, yet he seemed to not know exactly how it happened, and he actively avoided bringing it up.
At around the same time, a young girl also came to the orphanage. The pretty little girl always seemed so forlorn that the matrons dubbed her "Rue". For a young child, she was fairly aloof at first, even to Mytho, but she always seemed to hang around him, sometimes asking her to dance with him and displaying her notable talent for ballet that, once the caretakers found out about, decided to send her to dance school, to perhaps make her a more likely candidate for adoption by a well-off family that would nurture her abilities. The little girl refused to go, however, unless Mytho came with her, so out of desperation they asked Mytho to go with her and, wanting to make her feel comfortable, he agreed.
The two became fairly proficient at it, and each time they came back "home" they shared some of what they learned with Fakir. While Fakir pretended to be uninterested, sometimes they caught him practicing some moves on his own.
As it so happened, Fakir was adopted somewhat quickly by a local antique museum curator named Karon. While at first that gave them bittersweet feelings, that he could have a normal family but they couldn't see him anymore, their worries were dashed as they started school and realized they saw each other practically every day anyway. And somehow, Fakir managed to find time to visit them there, so it was almost as if nothing had changed.
One thing that set Mytho apart from the rest of the children, the caretakers found out over time, was that the boy did not age normally. He remained at the biological age at which he was first found, of approximately 14-16, which brought out questions of just exactly what kind of "child" he really was. Thus he remained in the orphanage for several years, taught by tutors within the house (as his primary school learning would have been very awkward at the age he appeared to be among all the younger children) until the day he moved out to attend school with people that at least looked his own age. Because of this, many of the children grew a bit irked about him and avoided associating too much with him, leaving his only constant companions as Fakir and Rue, who attended primary and secondary school with the other orphans but always returned to keep Mytho company. Also as a result, he spent a lot of time in the orphanage's, and later, the local library, becoming a voracious reader of books, especially fairy tales and folklore, which he took a particular liking to.
After some time, after Fakir and Rue had grown up to adolescence, at the orphanage there arrived another teenage girl with brilliant red-orange hair named Ahiru, also brought to the orphanage with (allegedly) no memories of her childhood, except for her name. Mytho and Ahiru got along very well and easily became close friends, although Ahiru sometimes clashed heads with Fakir; Mytho usually ended up needing to be the one to calm the two down. Rue didn't seem to speak much to the redhead, but she seemed on occasion to show her much gentleness, and Mytho even spotted them dancing together once.
A strange note, ever since he'd met Ahiru, Mytho had at some occasions spotted a duck hanging around the premises. He treated it kindly and fed it, and confided in it things that he was worried about. He had no idea why he kept coming upon the little bird, but somehow seeing it cheered him up.
Then, unexpectedly, the orphanage caretakers came to them one day and told them that Rue's family had been found! While her three friends were quite happy for the good news, Rue somehow seemed deeply troubled, and on the day that she moved out, she never smiled once. The three eventually started missing her as they realized that wherever Rue's parents were, it made her unable to come see them.
Some time after Rue left, the trio began hearing rumors about strange attacks on people during the night by mysterious dark beings that were nicknamed "Shadows", and all the victims that are unable to escape their assault are left in a emotionally comatose state, dubbed Apathy Syndrome by the professionals, and the victims called by laymen "the Heartless".
On one night of the full moon, the orphanage was attacked by a gigantic Shadow. As the caretakers and the older orphans attempted to evacuate all the younger children, the Shadow came for them. Suddenly, the trio each heard a strange voice in their heads, a voice that seemed their own and yet somehow different, and told them they had the power to save those people in danger, if they should choose to take them. As the trio desperately agreed, they transformed into supernatural beings with immense powers: Ahiru became Princess Tutu, Fakir became Sir Lohengrin, and Mytho became Prince Siegfried. In these forms, they battled the Shadow, and defeated it.
Immediately after the fight they all fell unconscious, and they all awoke in the hospital wondering if the whole thing had been a fantastic dream, until they found out they had all had the same dream -- and on the headlines, blurry photos emerged of three mystical beings battling a giant dark monster. Although they had been seen by quite a few people, no one apparently had any idea who the mysterious "Trinity", as they were dubbed by the media, of superheroes were. The trio was, needless to say, not sure what to make of all of this, and attempted (with little success) to put it in the back of their minds while all the chaos was straightened out. At around the same time, it was reported that all of the people affected by Apathy Syndrome suddenly recovered, seemingly with no explanation.
A few weeks later, now that the orphanage was in need of reconstruction and they were old enough to attend school living on their own, they moved out into the dorms of the private academy on the other side of the large town, where to their great joy they found that Rue was also to be attending school. Yet, when they came upon her, she seemed somewhat more reticent than before, as if she were hiding something. Still, she seemed happy to see them again, and they thought nothing of it.
Soon after school started, the headmaster Drosselmeyer called the three of them to see him. He tells them that all the strange attacks around town, resulting in the Heartless, were people's hearts getting stolen, and on the full moon of each month, like the attack on the orphanage, a great beast would appear, and wreck havoc on the town before disappearing at dawn. Shadows, he told them, were apparently made from those people's "hearts" getting stolen by other Shadows, and thus propagated; but, if they could be defeated, then their hearts would return. If the large shadow on the full moon were defeated, it would return the vast majority of hearts taken throughout that month. They then realized that, if they didn't want the town to be engulfed by the Shadows and all turned to Heartless, they would have to keep them at bay, going out to fight them at night to prevent them from taking too many hearts, as it seemed the more they took, the stronger the full moon Shadow was when it appeared.
Just a few days later, Drosselmeyer and the trio discovered someone snooping in on their meetings. A young man named Autor found out about their group and what their purpose is, and to keep him quiet Drosselmeyer allowed him to join them. While he seemed to have no alter persona of his own, he started doing much research on the attacks. They soon found out, after their first few battles, that the information he learned was of great use during combat, as from his data gathering (and being able to keep in contact through Tutu's support magic), he could often predict weaknesses depending on what the Shadow looked like, where they found it, and what its behavior was like, even making some educated guesses as to where they might find the strongest concentration of Shadows to defeat.
However, they discovered that the Shadows are not the only ones threatening the town. They come upon another alter persona named Princess Kraehe one night, and to their shock, she appeared to be summoning and directing many of the Shadows to their deeds! They didn't know who she was, but, while fighting her...Mytho suspected.
At school sometime later, Mytho confronts Rue privately, and while at first denying it, she told him to stay away and not go out fighting anymore. She doesn't want to hurt them, especially not him.....still, he couldn't get her to tell him much about it, only saying, after he promised to keep her identity secret from the others, that ("...if you knew why I was doing this, you would destroy yourself..."). Despite Mytho keeping his word, Fakir suspects he knows something. They are also getting increasingly worried about Rue who is spending less and less time with them, and eventually became absent from school altogether.
One night on the full moon, during a battle with a particularly strong enemy, Mytho's heart was stolen by the Shadow. In the process, Kraehe on the scene was revealed to be Rue. Rue found out about his heart being taken and is torn apart by the discovery. While Fakir had been suspicious of Rue from the beginning, Ahiru was crestfallen that Rue had kept her role a secret from them the whole time. She remains disappeared from school during this time, and this only makes Ahiru worry more about her and Fakir even more suspicious.
On the full moon after, a Shadow much more powerful than all the previous ones appeared. It is soberly attributed to the fact that of the hearts stolen, Mytho's is among them. This one will literally be a battle for Mytho's heart. During the bitter fight, Kraehe appeared and did a 180; she turned her Shadow-controlling powers onto the full moon Shadow, which enabled the weakened party of only Tutu and Lohengrin to defeat the exceptionally strong Shadow.
Afterward, despite having defeated the Shadow and Kraehe having stopped leading them (or really refusing to transform at all), the shadows became yet more persistent, and were even starting to claim a few victims during the day. And, after Mytho had gotten his heart back, he'd been having troubling dreams at night, dreams in which another voice like his own but not like Siegfried's invaded his mind and threatened to make him lose control of himself...
Autor, all the meanwhile, (although, he too, upon discovering Rue's secret, kept it from the rest at her request...) has been uncovering under Drosselmeyer's nose some disturbing things about the Shadows and their attacks, secrets of the town that they live in, and in the process, parts of his own team members' pasts...
Mytho/Siegfried is taken from sometime in the month after Mytho got his heart back, Rue having not yet told them what she knows or has been doing in the past several months but feeling deeply guilty for having fought against her own friends and hurt one that she loved the most.
The setting is technically modern, but the town in which they live seems to be kind of set off from the outside world, leaving a very refined, antique atmosphere to the place. Having an old-fashioned style, modernization is limited there, with cell phone reception, a television broadcast station, and some internet access being some of the few high-tech conveniences available.
Despite this being quite comprehensive, there is a lot of other backstory to the characters not mentioned, like what happened in the earlier childhoods of Rue, Ahiru, and Fakir, and what happened before Mytho lost his memory. Of course, Drosselmeyer has a lot going on that hasn't been revealed to them. Believe it or not (and because when I thought of the past story it was too interesting to resist), the half-canon, half-OC character Caras aka the Raven in human form (of whom there is a 'canon' version of him RPing as menschlicherabe) is in this universe too, although in a markedly different context than he is set in the 'canon' universe. I'll just say that, in the part of his memory that was lost, Mytho and Caras were not enemies but friendly rivals. Still, as you've noticed, there are many, many parallels from the characters here and their original counterparts. Besides most of the side characters appearing as students at the school, Edel also appears in this universe as Drosselmeyer's secretary, and Uzura as her tagalong 'baby sister' that often pipes in randomly at the Trinity's meetings. No one's inquired as to their background, however, and Autor, from his snooping, still suspects something strange about them...
If by any chance someone is interested in playing one of the other characters from this universe, PM this account or tomoyoichijouji and we'll talk! I have a lot of it thought out already, but if you've got some of your own ideas for backstory I'd love to hear them and see what we can do with 'em.