Tsukasa Kuze is the younger brother to Azusa Kuze. 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 is brothers spirit. Tsukasa does eventually talk to the MC, 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 the MC, 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.