Gaku and Yura are twins, so they look similar to each other...but their personalities and the things they like are vastly different (something Kuya complains about). This reflects their origins as them both being instruments, yet different types that produce a different type of music.
A flute is purely for melody, and a drum provides the beat, to be exact.
It is revealed that Aoi was the person who loved you a century ago, and that every time you choose someone other than him (which was also the case back then), he'll have to relive the entire thing again. No wonder why he thinks that everything with a heart will betray him. No wonder that he is consciously pushing away any kind of ideas pertaining to loving you!
Aoi: ...who cares what I do. She's got that ogre or whatever.
Aoi: I'm not like Koga.
Aoi: I can't be fearless. I can't laugh it all off...I don't have that kind of strength.
Aoi (in thought): (MC) needs a guy like him.
Aoi (in thought): Those two have always been perfect for eachother.