"Níl" is the negative form of "tá," which is the present indicative tense of the verb "bí" - to be. "Ní" is the negative form of "is," which is the copula. You use the indicative tense to describe a state, condition, position, location, or appearance of a subject (e.g. Tá sé tuirseach) or in conjunction with another verb to describe what a subject is doing (e.g. Níl sí ag snamh). You use the copula ("is" for positive or "ní" for negative) when you are equating two nouns (e.g. Is fear mé). In this case, he and enemy (two nouns) are being compared so the copula is used. I know others have described this better if you look back through some of the other conversation threads, but I hope this helps!