Minatsuki, Saya

A Sweeper who found and helped Train when he was injured. She introduced him to a new world, so to speak, and helped him become the man he is now. Their relationship was deep, and was not romantic (According to Train), but may have been if given more time. As the manga progresses, we learn more about Saya and her relationship with Train, as well as about Train's childhood and traumatic past. Her weapon of choice in both the manga and anime is a Beretta 93R, and her special technique was ricocheting bullets at different angles and velocities.

* In the Anime:

Saya appeared suddenly to Train on a rooftop one night, and the two of them shared a strange friendship that spanned only a few days. Still, those few days seemed to be enough to cause a change in Train. Saya influenced Train to do what he wanted to do and live freely, which causes Train to decide not to follow Chrono's orders anymore, as he leaves. Creed, who admired Train's way of killing and felt a paranoic obsession with Train, watches from afar until he gets the chance to one day meet with Saya when Train's not around, and kill her. Except for the timeline changes, the situation between Saya and Train is pretty much the same as in the manga.