Aoi is a laid-back, cool-tempered man, and the father of Mitsuki. He was the leader, guitarist, and wrote the lyrics and composed the songs for the legendary band 'Route:L'. Aoi had a very close relationship with his band mates and friends, especially Takuto Kira, who was an orphan and viewed Aoi as a father figure.

Spoiler

Note: In the manga, he dies in an accident while trying to get Mitsuki's mother, Hazuki, to the hospital when she was due to give birth. In the anime, however, he dies when Mitsuki was one year old.

(Source, Wikipedia, edited)