Patroclus was the son of Menoetius. There are several women who could be his mother, because the different stories and writers are not sure. He was the best friend (or possibly lover)of Achilles.
In the Trojan War Achilles once stopped fighting because he was angry at Agamemnon. At this time Patroclus secretly took Achilles' armour and weapons and went into the fight. All people mistook him for Achilles and he was killed by Hector. This made Achilles angry at the Trojans so he started fighting again.