See the writeup on Japanese Pronouns.

Kimi is a problematic pronoun in that it doesn't necessarily have to be an informal, or particularly rude. You find it in poems. Song lyrics often use kimi. Military people and soldiers refer to each other as kimi.

Kimi is also a suffix that is appended to names to indicate rank; a daimyo, or a prince would have it. In this case it has a different kanji, but the same meaning as the pronoun.

The Chinese word from which it is derived, Jun1, means noble, or a prince. Jun zu meant noble ruler. Its translation into Japanese probably retained this meaning, but gradually came to mean the same as all the things which the Japanese kimi refered to.

The kanji for kimi is also -kun, an mode of address appended to names.