Takeshi is a long time friend of Savitar, Acheron Parthenopaeus, and others. He has a take-no-prisoners attitude. He just kills the ones around him. Savitar claims he's the one who invented war. He is lethal, quick, and older than dirt. He is a powerful Zeitjager, one of the guardian forces which monitor those who mess with the flow of time. He has since "retired" after bargaining for freedom.
As a Zeitjager, Takeshi does not experience time fluidly. He is able to live at different points in a nonlinear order. As a guardian of time he is also able to see paths of fate and changes to them, though since retirement he has taken a less active role.
Don't be fooled by his calm exterior; it just means he's sizing you up for a body bag. Along with Savitar, he trained Acheron after Artemis brought him back from the dead. Takeshi makes his first appearance in the book Acheron.