God of the sun and master of some really awesome cattle that no one was allowed to touch, much less kill and eat. When Odysseus' men do just that, Helios demands revenge, basically by holding his breath until Zeus agrees:
Father Zeus, and you other everlasting and blessed gods, punish the companions of Odysseus, son of Laertes; for they outrageously killed my cattle, in whom I always delighted, on my way up into the starry heaven, or when I turned back again from heaven toward earth. Unless these are made to give me just recompense for my cattle, I will go down to Hades' and give my light to dead men. (12.377-383)
For more on this guy, check out Shmoop's guide to Helios.