Achilleus (or Achilles) is a big Greek hero who fought against the Trojans. He's already dead by the time the Odyssey begins, so we only see him in the Underworld. In the Iliad we hear Achilleus had a choice between living a long and uneventful life, or get lots of glory and then die. Obviously, he chose the latter—but he's not happy about the decision. He tells Odysseus that he'd rather "follow the plow as thrall to another man, one with no land allotted him and not much to live on, than be a king over all the perished dead" (9.489-91).
For more on this guy, check out Shmoop's guide to Achilles.