How convienient, let's not reveal anything (or very little) about a character, call it a mystery, and then make him capable of anything. I wager, by Uncharted 5, Drake will be able to fly, cross dimensions, and will have clones of himself fighting over control of the universe.
Backstory is NECESSARY in creating a stable story/plot. Otherwise it becomes a make-it-up-as-you-go-along type of story, which becomes hit or miss. I'm not saying you need to reveal EVERYTHING, but there needs to be something to explain his uncanny ability to kill (trained soldiers no less).
It's not a matter of realism, its more a matter of plausible fiction. But hey, if your blind fanboy-ism can suspend your disbelief THAT much, all the power to you.