Pirithous, Prince of the Lapiths, was Theseus's best bud. From the moment the two heroes set eyes on each other, it was a total bromance. We're guessing they probably spent a lot of their time together going, "You're cool." "No you're cool." Anyway, the point is muck like Achilles and Patroclus, or Heracles and Iolaus, these two were inseparable. The hero buddies went off on lots of adventures together like the hunt for the Calydonian Boar, and the Centauromachy, a battle against Centaurs.
Why did they get along so well? Maybe, it was because they both had godly fathers on top of having mortal ones. After all, Theseus was the son of Poseidon and Aegeus, while Pirithous was the son of Zeus and Ixion. Of course, they might have been such good buddies because they both had the same attitude towards women. Both heroes seemed to see them as objects to be taken, which wasn't exactly a minority opinion among the males of ancient Greece.
Pirithous's misogynist attitude actually eventually causes his downfall. When he enlists Theseus to whisk away Persephone, the wife of Hades, Lord of the Dead, only Theseus makes it out of the Underworld. Pirithous is doomed to forever be frozen to a rock, or tied to a chair by snakes, depending on who you talk to. Either way, much like his father Ixion, who is forever bound to a fiery wheel, Pirithous is doomed to a pretty unpleasant afterlife. (O.K., having to sit in one place forever is probably not as bad being tied to a fiery wheel, but it has got be seriously boring. It's not like Pirithous can't do Sudoku or anything.)