Pirithous, Son of Zeus. King of the Lapiths

The story goes that Pirithous, having heard of Theseus' prowess in battle, wanted proof of his courage and strength, and so, as all good heroes do, he went out to rustle some of Theseus' cattle in order to test him. A time honored tradition among demigods! Plutarch tells the rest of the story in his essay on Theseus:

[...]when the news was brought that Theseus pursued him in arms, he did not fly, but turned back and went to meet him. But as soon as they had viewed one another, each so admired the gracefulness and beauty, and was seized with such respect for the courage of the other, that they forgot all thoughts of fighting; and Pirithous, first stretching out his hand to Theseus, bade him be judge in this case himself, and promised to submit willingly to any penalty he should impose. 

And that, as they say, was the beginning of a beautiful friendship. From that day forth, Theseus and Pirithous were like brothers, allies in everything and as you might recall from Ovid's Heroides, rather inseparable. Of Pirithous alone, we have very little information. Most of what is preserved is only in relation to his adventures with Theseus, in which, to my prejudiced readings, he comes off as the instigator of some foolish endeavor, inevitably resulting in trouble for both of them (the primary example being the abduction of Helen, and the subsequent trip to Hades so that Pirithous might steal Persephone for his own bride, because that is just the kind of overconfident, arrogant piece of work that Pirithous was).

Which guy is Pirithous and which is Theseus, I couldn't say.

But the most famous story of Pirithous is the story of his wedding to Hippodamia, which unfortunately for everyone involved was something of a disaster. A good summary (still Theseus-centric, but what can you do?) of the story was preserved in Apollodorus' The Libraries [E.1.21]:

And Theseus allied himself with Pirithous, when he engaged in war against the centaurs. For when Pirithous wooed Hippodamia, he feasted the centaurs because they were her kinsmen. But being unaccustomed to wine, they made themselves drunk by swilling it greedily, and when the bride was brought in, they attempted to violate her. But Pirithous, fully armed, with Theseus, joined battle with them, and Theseus killed many of them.

Not exactly an idyllic occasion. I imagine that Pirithous had some trouble from the Centaurs from then on, after slaughtering so many of them. I also imagine he had a very, very, very unhappy bride that night. Pirithous strikes me as the kind of man who takes advantage of weeping women by letting them cry on his shoulder and then copping some feels on the sly, and I highly doubt that this instance was any exception.

But don't misunderstand me! Pirithous might be something of a cad, but who can resist the scoundrel who is also a hero? True, we don't really have much information on any actual HEROIC deeds, outside of rescuing his wife from the centaurs, but it's clear that Pirithous was along for the ride with Theseus often enough that he must have done SOMETHING honorable to make a name for himself. It can't all have been abducting women and cattle rustling...

Then again. Maybe Pirithous was more pirate than king, after all.