Okay so I've nerded out about Vocaloid here before, but not I have an even better reason to. 'Aku no Musume' is part one of the oldest and most popular Vocaloid series ever. It's part of the Subseries "Story of Evil", and a major part of the overarching "Evilious Chronicles" series. It's so popular on its own that it's not even funny. If you know Vocaloid at all, you've at least heard OF Daughter of Evil.
This is a long post, but I really want to share at least the story of the musical because I really want to see how Reina handles Riliane's character. Read on and see why! 'v' (Spoilers below, obviously)
Princess Riliane (who Reina's playing) is a selfish, spoiled ruler of the kingdom Lucifenia. She executed people for small offences and taxed her struggling subjects, acting in a very "Marie Antoinette, let them eat cake" way, all of this seemingly because she's young and immature.
When Prince Kyle of the neighboring kingdom of Marlon breaks off their engagement for the sake of a green-haired peasant girl, Riliane enacts the "Green Hunting", the capture and execution of all Elphe women (who had green hair). As it turns out, Riliane's servant (secretly also her twin brother) knew where this girl, Michaela, was being kept. He was also in love with her, having accidentally met her before the dissolution of his sister's engagement and the Green Hunt. Riliane orders him to kill Michaela to prove himself to her. And he does, because he is utterly devoted to his sister.
This sparks a war with the Elphe people and a revolution of her own citizens. Riliane is unconcerned, and is occupied with her own fancies, such as snacks and horse riding lessons. She doesn't acknowledge the situation until the palace is under siege. Allen quickly switches clothing with her, to her confusion, and reveals their shared blood. She doesn't understand why he would take her place, and he remarks that she is an evil girl, but they share the same blood; thus he is evil too and will serve well as her replacement. He helps her escape through a secret passage, leading her to safety as he takes her place.
The story of Aku no Musume ends with "Riliane"'s execution as "Allen" watches from the crowd. In that moment Riliane breaks down as the crowd cheers her supposed death and she loses Allen.
Now this is the basic story. The full Evilious Chronicles reveal that actually Riliane was possessed by the Demon of Sin Pride and that's why she was so utterly uncaring. And in fact, Michaela was a lesbian and rejected Kyle, who only broke off his engagement to Riliane because the Demon of Lust was influencing him to want Michaela no matter what, oh and also later on the Demon of Pride moves on to Kyle once the Demon's Vessel is given to him after the war ends, and he sparks another war, etc, etc.
Oh, also, there's this whole reincarnation thing happening for pretty much everyone involved, and in the past the demons possessed their ancestors, and there's also this woman running around who's like a thousand years old and she has two demi-gods sealed inside her, and the spirit of Sloth keeps reincarnating itself as dead girls, giving itself amnesia, and thinking it's actually those girls all the while there's a genocidal god-scientist hell-bent on releasing a devastating virus on the world (for the third time, the gods are sick of having to move on to more worlds Seth, give it a rest).