As mentioned, you use 아가 to call a baby ("아가야..."). Sometimes older people call their daughter in law (their son's wife) 아가 for endearment. AFAIK between lovers or friends, they don't use it like "baby" in English. People use 아가씨 for young women they are not familiar with like "miss" in English. This is usually for addressing a stranger or an in-law (brother's wife), never between lovers. The casual endearing term between lovers is usually 자기 ("oneself", "yourself").