Who was Achilles' lover?

Can you clarify for me, who exactly was Achilles' lover in Greek mythology? I'm aware of his fame as a formidable warrior and the protagonist of the Iliad, but I'm not entirely sure about his romantic life. Was his lover a mortal woman, or did he have a relationship with a divine figure? If possible, could you provide some details about their connection and its significance in the mythological narrative?