Two adjectives that could describe Achilles are sulky and temperamental.

Achilles sulks in his tent because his sense of honor is offended when Agammenon takes Briseis, a young woman, to be his war booty when Achilles thought she was to be his prize. Even though he is Greece's greatest warrior, Achilles sits in his tent and refuses to fight. The fighting is going badly without him, but he cares more about his personal sense of injury than the wider issues of warfare.

However, Achilles is more than just a sulker. That is just one aspects of a wide range of emotions or temperaments he exhibits. His is a fiery and volatile personality. For example, when his best friend Patroclus dies in battle, Achilles is galvanized into action by grief and anger. He joins the battle and kills Hector but doesn't let that satisfy him until he has dishonored Hector's corpse by riding three times around the walls of Troy, dragging Hector behind his chariot.