He was born overseas in Chiyoda, Japan, both his mother and father working on an American military base before transferring over. His mother announced her pregnancy once they were settled in and began to immerse themselves in the culture there. While being a "white Japanese" is either taboo or not believed in, this character is fully fluent in Japanese and attended the local schools until he was old enough to decide on his future for himself. However, he is a white male and would not have a Japanese name. He is quiet, set in the ways he will be given, and was told that he should grow up to be like his parents and work in the military. Unfortunately for them, he used any further training for private companies seeking contract killers. Something of the sort. He believes he is doing them a mercy favor, or is a mercy killer, truly believing he is the only one who can decide the fate of people who want to put money on a target. Needless to say, he probably lives well but receives no psychological or emotional evaluation from his peers. He will only have one romantic interest, with a younger man, and that is yet to be developed. If he's to be given a name based on his looks, you can imagine Jeffrey Dean Morgan as either Negan or the Comedian.
Nothing comes to mind, personally, but you could try this site: Japanese name generator http://fantasynamegenerators.com/japanese_names.php#.VwMXW0ewXBA
The name Lennon came to mind, before I got to the second paragraph about him. Then I got Cam (either Cameron or Camden) before the physical description. Now I'm thinking he needs something short and sweet: Abe, Dean, Opi, maybe even Perry. Hope that helps some.
Seth is the first name that popped into my mind. Seems like a perfect fit to me. If you're like me and obsess over name meanings it means "Anointed, or compensation"
Nick, or Nicholas. Maybe Nicholas Ryder? Or Ryder as a first name. I like Dean Morgan, but not Jeffery Dean Morgan. Maybe Josh Holliday?