Kazuki seems at first like just your average, unassuming main character, but he's got a genuine kindness and sincerity that's not typical of a shounen hero at all. Sure, he turns out to be an amazing fighter when he's unceremoniously tossed into the middle of a war he didn't know was raging, but fighting is never a thing of central importance to him. Like most of the children on Tatsumiya Island, he had no idea the earth was under invasion and strives throughout the series to understand why the adults hid this from him, and especially, why his estranged childhood friend Soushi is so willing to do whatever it takes to defend this humble island. Naturally, to someone with such a gentle nature, having to fight for his life constantly is a bewildering experience that causes him to question who he is and what he believes in, but he really grows as a person over the course of the story and becomes everything a real and uncliched hero ought to be.
Kazuki is originally the pilot of the Mark Elf (XI), but is given the much more powerful Fafner "Mark Sein" later in the series. His mother, Akane, was assimilated by Festum long ago and he now lives with his father Fumihiko. He enjoys getting to know Soushi outside of battle again, and feels comforted when he's with Maya. Living with only his father, he does all the cooking and shopping and thus is an excellent chef. He also seems to be very bad at card games (he's too honest to be good at bluffing).