Tsukasa died from an incurable illness year(s) ago, leaving his brother alone. Azusa himself believes that his brother can be brought back to life however, launching his motives throughout his story.
Tsukasa is also the reason why Azusa is seen wearing a large pendant around his neck. Made as a lasting memento of his love just before he died.
During Azusa's story the Nue takes Tsukasa's form, further tricking Azusa into doing what it wants to bring his brother back from the dead. This is impossible however, as the Main Character finds out, and soon discovers that it is, in fact, not his brother's spirit.
Tsukasa does eventually talk to you, however never showing his true form in the very climax of the story line. The form he chooses is what you see through out the story as the mysterious golden butterfly that talks to you, giving you second chances should you pick the wrong options.
The butterfly is also seen in the pendant around Azusa's neck throughout his story. A visual tell of Tsukasa always being there and watching over his older brother.
He also appears in Vincent Knight The Sequel as one of Devi's minions. He shows up to Liz, Vincent and Klaus during a Ministry investigation without any knowledge of who he is or how he got there and that he only remembers his name and that he has a brother. Liz tries to help him, but neither Vincent nor Klaus seemed to believe he had amnesia, and were prompted to believe he could be behind the monsters attacking both, the Academy and Gendolune town. However, Liz doesn't think he is either dangerous nor untrustworthy, seeing him as a lost little brother.
Tsukasa also seems to trusts blindly in Liz and is always happy to see her. Vincent gets jealous of Tsukasa for a moment, thinking he might be developing feelings towards Liz, but Tsukasa was seeing Liz more as a family figure, at the beggining as an older sister, and later as a mother.
In the end of the sequel story it is learned that he was a doll created by Devi using the soul of the late Tsukasa. This is why he couldn't have memories of his present "life" and also it is discovered that because of this, he had to do things he didn't want to do, including attacking Liz and Vincent. When he rebels against Devi she dismisses him and destroys his new body. Liz suffers Tsukasa's death but Tsukasa, on the other hand, is happy that he is no longer used for evil purposes especially since he didn't want to keep hurting Liz. Before he died he mentioned that his greatest wish was to be reborn as Liz and Vincent's son.
After he disappears Liz rises up in revolt against Devi and the other creatures she was controlling. Although Devi manages to escape Liz remains frustrated for not being able to save Tsukasa, but Vincent consoles her. In the happy ending it is revealed that Tsukasa wish came true which is the ultimate happiness for the couple.