also: なかがよい
expression, i-adjective (yoi/ii class)
close; intimate; on good terms
Describes a friendly, harmonious relationship between people. Often used with と to indicate the other person.
彼とは昔から仲がいい。
I've been close with him for a long time.
あの二人はとても仲がいいね。
Those two are really close, aren't they?
親しい is an adjective meaning 'close' or 'intimate,' often used for personal relationships. 仲がいい is a phrase that specifically describes the state of a relationship between people.
仲良し is a noun meaning 'close friend' or 'good relationship.' 仲がいい is the adjectival phrase describing the relationship itself.
Compound of 仲 (relationship) + が (subject marker) + いい/良い (good). Literally 'the relationship is good.'