There's no kanji for it. It's a short form/plain form verb ending. Ja nai = ja arimasen. Negative non-past tense ending for nouns and na type adjectives. IE genki ja arimasen = genki ja nai = genki nai = not healthy/energetic/etc. All depending on the level of formality, of course. If I'm not mistaken, it evolved from dewa arimasen.
-Karrde-