A troubled soul. The commander of TERRA's tactical division, Jin Kunugi is probably one of the more complex characters in the series. The impression we got when he was introduced is that he was simply a bossy cold hardass that had particular dislike for Ayato. As the series progresses though, we find that Kunugi is a very melancholic individual. When he speaks he usually doesn't have much to say, and when he does it means serious business. He is friends with Watari Shirow and Rikudoh, other than these, he only opens up to Quon, his personal music performer, and Souichi, his right hand, he rarely speaks openly to anybody else. He starts to warm up to Ayato later in the series when Rikudoh tells him to give the kid a chance.
Kunugi's story is probably one of the more interesting ones in the anime because it's directly tied with the Great Mu War. When he was an officer in the military, he was under the command of Kuki Masayoshi, an under-cover Mulian sent to engineer a fake war. Kuki ordered Kunugi to execute an attack againts the Mulians even though the Mulians weren't attacking; actually, they weren't even communicating. If Kunugi didn't execute the attack, he was to be court-marshalled, and so he had to do it. However, this attack had disastrous consequences. The Mulians retaliated by sending one of their most powerful dolems, Ritardando, and ended costing the lives of over six million poeple in Australia alone. Among them were Kim Hotal's paretns and his daughter Michiru. After these set of events, Kunugi was blamed for the war and was consequently kicked out of the military. Even his wife decides to leave him since he blamed their daughter's death on him.
Some time later Watari Shirow convinces Kunugi to join Terra and fight againts the Mu. Kunugi accepted since after all he had nothing else to lose. He swore revenge on the Mu and essentially hates anything that's Mulian, including Ayato who is pretty much unaware of the Mu's intentions.
Years after the Great Mu War, Kunugi still visits his daughter's grave on her birthday. In the scene shown in the anime, Kunugi had finally been forgiven by his ex-wife and also got the missing piece of a musical score his daughter was composing, in which Quon congratulated him on his achievements. At the end of the series, Kunugi finally confronts Kuki and kills him with a suicidal attack, taking all of Terra along with it.