Aoi Koga 古雅葵

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.

In the anime, however, he dies when Mitsuki was one year old.!~

(Source, Wikipedia, edited)