Justin Bieber Cantor.

Edvard Munch Hallifaxian.

Hard to sustain when we do have reflective surfaces like ponds and mirrors.

A bit harder maybe and it might make the belief a bit less realistic, but there have been stories and movies that tackled a similar situation. The power of belief/denial is a strong one indeed. If the character truly believes it, then they might refuse to recognize and keep their beliefs, even when shown their reflection.

He or she could be pathologically in denial of such facts even when confronted with them, and might cast the distortions on the external factor rather than on themselves. An igasho who is convinced he is a furrikin may tell himself that the image is distorted into looking larger by ripples in the ponds, or maybe a merian who insists she is a dracnari chain smokes to convince herself that she can breathe fire (or at least smoke).

Even better with mages, as they have changeself. They may have convinced themselves so thoroughly that they of that race that they do not notice that they've weaved such an illusion about themselves.

I know a guy that does that.

I'm not saying you couldn't do it; I'm just saying, it is very likely to get very unfun, because almost everyone around you will brush it off.

Generally speaking, you'll get a lot of kudos for very gimmicky roleplay (that's usually the only type that gets recognized, for various reasons) up to a point, and past that point, when it's too gimmicky, it's likely to backfire after only a short time. This seems like the latter.

Ah, I interpreted 'hard to sustain' as hard to pull off, instead the long-term identity you were talking about.