as a rule of thumb, you should not create a concept that does not already exist in the universe you are using. Creating a half elf half human character in suikoden sounds terribly marysueish to me, no offense, I'm not targeting you necessarily, I just give you this advice: it sounds suspiciously contrived. Just ask yourself the question: do your character really needs to be half-bred? Is it really fundamental to its identity? Couldn't it be played in a more simple and natural way instead? Deep inside, aren't you really just the only one to enjoy that concept. You ask yourself these kind of things constantly when you write a story. And I think the answers in this case will tell you it's not a good idea.
edit: to be more precise, by definition, any character you create that can be questionable should not be created. Characters cannot afford to be questioned. They may not be universally liked, but if it is debatable whether they are plausible or not in their universe, then they should be discarded or redesigned.
Look, I hate Pesmerga, but I don't think he is out of place in the Suikoden universe. The idea of a black night that's some sort of mysterious being chasing another similar character is not something out of place in Suikoden. You will notice that his true nature is blurred, which makes it more acceptable in the framework of the suspension of belief that applies to anyone accepting to follow the storyline of a fantasy universe like Suikoden. In a way, I don't mind Pesmerga as a secondary or third-class character, a blank filler of sorts. What I don't like is how people overreact to such a footnote in the game. But as a footnote, he's acceptable, there is no matter of debating whether he could exist or not in Suikoden. I have less of a problem with the Suikoden team creating him than I have with his fanboys.
Working on the rewriting of SIV, I recently had an idea for a plotline that I explained to sticky. Sticky's reaction was so-so, something like "yeah why not but I'm not sure about this and that fitting with the game". My reaction was simple: I just dropped the idea. By definition, if my test audience was not enthralled by the idea, then I should simply come up with a better one.
My wife and I are writing a screenplay for a comedy. We pitch jokes to each other. We work on a very simple way: unless she bursts in laughter at one of my jokes, I throw it in the wastebin.
If you are a creative person, you will come with ideas all the time. If one of your ideas is questionable, just don't keep it. You'll find another, better one.