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.
Kuro's interest in foreign cultures. He wants to learn English and is used to customs very different from Japanese ones, which makes sense since he's a nue, which is made up of very different animals!
Kuro's favorite story happens to be Beauty and the Beast. Think about the nature of his relationship with the protagonist...
The fact that Oji gives himself up to be arrested and likely executed in Akiyasu's place near the end of his route and doesn't seem too bothered by it. Is he a Death Seeker? Did Asa's disappearance cause him to become severely depressed? Thank goodness everything ends well.
