Once upon a time, about nine months ago, I started watching a new television show that had become quite popular. After observing one of the characters for several episodes, I came to a simple conclusion: If I hadn't gone to high school with that guy, I wouldn't have believed him as a human being.
Now, I didn't go to high school with the actor nor the fictional character. When I went to high school, I very much believed that young man as a human being, albeit a human being in a v-neck t-shirt. However, watching that T.V. show, I came to the conclusion that without that friend from high school, I'd have decided that T.V. character was absolutely ridiculous. I checked with some mutual friends, and we'd all had a very similar reaction.
I do believe that there are a great many true things in this world that no one would believe if they didn't see it in person. (I'm not claiming there's any sort of realism criterion for this. It seems to be at random. True things I've seen people not believe: My cousin once taught a student named Dragon Boat; Canada is not a barren wasteland; Hawaii is a part of the US. False things I've seen people believe: Canada just changed from the 25-hour clock to a 24-hour clock; everyone in Canada travels by dogsled; my sister and I speak 5 languages.) However, I do know for sure, one must make sure that people who aren't you and don't have your background believe your characters.
Ask yourself, is your character believable to a regular audience. How about that character who speaks with a British accent for no explicable reason? Or the character whose parents tricked him into thinking they were going on vacation in China for years when they were actually moving there? And the character who responds compulsively to certain sentences in a language they don't actually understand?
Will a reader believe any of these people? That's the question you've got to ask yourself. Because even if you've met a person just like that in your actual life, (and I've met all the aforementioned people), if that's the kind of person no one really believes exists, you're going to run into a problem.
Have you ever written a character people didn't believe? Have you ever run into such characters in books or T.V.? Have you ever met a person you almost couldn't believe?