Reiji Kizaki/Plot

Background
9 years ago, Kizaki's father was killed while protecting a child as a rescue worker. This death inspired Kizaki to make himself stronger due to the doctrine his father implanted into him: "rescuers who don't return alive fail."

Black Trigger Capture Arc
Kizaki first appears when Jin and Karasuma try to trick her into thinking that Osamu, Yūma, and Chika were his siblings. He dismisses the joke. Later, he is assigned to Chika a mentor. When he has her go for target shooting, he is immensely shocked at the amount of Trion she has. He also helps Osamu train physically, as fitness was important for fighting in a Trion body.

Border Enlistment Arc
He does not appear in this arc, but Azuma mentions that he had a lot of faith in Chika to have taught her that Snipers need to run after shooting their target.