Yagami Arian was originally a soldier under Nelle Poe who fought for Mars independence during the Pan-Galactic War, where he was apparently part of a vast battle in which he was either the only survivor, or one of very few. He is nicked-named "The Reaper" because of the many RBs he has destroyed, and the coldly efficient way he operates; early in the series, he informs Elizabeth that "I just don't know how to hold back."
Yagami initially has a cold demeanor though he isn't adverse to a good practical joke, and at one point stages a faux mutiny as a prelude to Ester's birthday party. Though he rarely shows it, he has great affection for his crewmates, particularly Gram and Junior. However his loyalties are somewhat split, since he has served under Poe for many years yet feels great loyalty towards Elizabeth. Eventually Yagami chooses to return to Poe's side, and leaves Elizabeth a valuable piece of intel so she can find the Stone of the Gods. He then takes off in the Vector of Wings, leading the Earth Forces RBs away from the Ship of Aurora. However when he discovers that Poe has sold out the Ship of Aurora and that her crew will be executed, he returns to his true friends' side. [wikipedia]