Samael

A quiet and analytical hitman. Grew up an orphan who unfortunately stumbled upon a murder-for-hire. Although he attempted to run, he was easily caught by his future mentor, who spared his life and actually took him in. He learned how to defend himself as a necessity for living with his mentor, eventually being driven to kill someone who had come after him. As his mentor quieted down and retired from his profession, Samael took over his role for his clients to support the two of them.
Eventually, he meets Xion in a similar way he met his mentor, but Xion was interested in the business and offered to manage his clients for him. They make a weird duo but work together well.