In reverse order:
Like someone who had practiced until they got it right. When you’re training, how good you were at the beginning doesn’t matter when you’re done. If you have the dedication to commit, and push yourself you can do pretty much anything. You practice until you get it right. That’s where the dedication comes in. It’s about not giving up until you’ve learned.
An unremarkable student who commits to the training, puts in the time, and has a drive to “get it right,” can become a formidable fighter.
In comparison: a talented student who views training as a distraction, wants out as fast as possible, and thinks their starting point is “good enough” is doomed to mediocrity.
Just because you don’t have a natural aptitude for something doesn’t mean you cannot do that. It just means you’ll need to work harder. If you’re ready for that, then initial talent is no obstacle.
Okay, on to the first question:
Physiologically? Yes. Psychology? Maybe not so much.
It is physiologically possible. The throat is comparatively soft tissue, and if your mouth is right there, against a foe who is otherwise distracted, it’s a relatively easy target.
Failing that, the nose, ears, and lips are also vulnerable in this kind of a situation. But, it is entirely possible to bite a chunk out of an attacker.
The real question is, “can your character get to a place where they’re willing to kill someone with their teeth?” For most people, that answer is “no.” This is a state of mind that runs contrary to all of your social conditioning.
Biting someone else is a behavior that has been conditioned out of you since childhood. Everything you’ve been taught about acceptable behavior says, “this is not okay.”
For example, ask yourself, “could I bite that guy over there?” I don’t mean as a gag, for fun, or foreplay. I mean, can you look at that human being as 200lbs of ambulatory meat? Not just at an intellectual level, but in such a way that you could just rip pieces out with your teeth?
For most people, the answer is, “no.” They can’t force themselves into a mindset this feral. They won’t even consider it as a possibility. Maybe as an intellectual exercise, but not an option in the heat of the moment.
As I’ve said before, this is some pretty messed up behavior, with some very serious social consequences and medical risks. It can be done, but it’s something you need to build towards when creating your character.
This can work with a character who is feral in their own right. Either because they were never exposed to civilization, or because they’ve chosen to reject it. A D&D style barbarian or a character with some kind of animalistic approach. Expanding on fantasy concepts, a character who magically transforms into animals (D&D Druids and Rangers, or lycanthropes) could also probably get there pretty easily.
A character who is incredibly opportunistic, brutal, and disciplined enough to fully disregard social norms when it suits their purpose could work.
In either case, this will cause others to view the character as little more than an animal. In the case of the later, you’re informing your audience that said character is more of a trained attack dog rather than a person. In the former case… you’re telling your audience and anyone who witnesses the attack that your character is (at least partially) more animal than human.
Just remember: If one of your characters is willing to bite their attacker, this is sending a permanent message to your audience about who this person is. If you want that, then this is an excellent way to send a message. If you don’t, then this is probably something you should avoid.