amicus

Amicus, amici, m., second declension, straight from amo: the one who loves. The vocative is amice, regular for a noun in -us, and it is common enough in dialogue to be worth learning as a form in its own right.

The same word is also an adjective, amicus, amica, amicum, "friendly", and as an adjective it governs the dative: rebus nostris amicus, "well disposed towards our cause". Be careful with the feminine noun amica, which means "girlfriend" or "mistress" at least as often as "female friend", so it is not the neutral counterpart the ending suggests. In diplomacy amicus was a formal title: a foreign king could be styled amicus populi Romani, and that carried treaty obligations, not warm feelings.