Hmm, I see it as Akihiko feels guilty over his sister's death. She was his only family and despite the fact that he was but a child and couldn't do anything for her, he believed he let her die. That he was responsible for her death. He says he started his obsessive training in order to prevent that from happening again, but it always struck me that he never got over what happened to her and this was his way of making up for his "crime." He would train and get stronger and protect everyone else, not only to make up for his inability to save his sister, but to punish himself as well and perhaps it was also his way of dealing with the grief. That's why he feels the need to protect people who don't necessarily need his protection.

Akihiko seems had an obsession with the past, I've noticed. Obviously, with the fire and his sister's death, but also with Ken and Shinjiro. He identifies with Koromaru's need to honor his dead master, which is why he was so desperate to help him. For months, he kept bothering Shinjiro, looking for that "old" Shinjiro Aragaki who would traverse the Dark Hour with him and Mitsuru, eliminating shadows and looking for answers. If I'm remembering correctly, when Akihiko tells Shinjiro to let go of the past, Shinji calls him a hypocrite, showing that Aki himself had as much attachment to the past as Shinjiro did. He used Ken as a means to convince Shinjiro to rejoin SEES. He was the one to first realize what significance October 4th meant to the two of them.

That's all I can think of for Akihiko right now. ~_~ I'm sure I'd have more but ugh, stomach ache.