We know that a lot of twins hate being lumped together, but we just don't think the Dioscuri would mind. These guys are so inseparable that they'd totally be cool with sharing a character analysis. Really, there's not much difference between the two of them. Both are athletic, and are great warriors. Some distinguish them a bit by saying that Castor was especially good with horses and Pollux was the one who excelled at boxing. However, other sources say they were both good at both things, so whatever.
The one major difference between the twins is that Castor is mortal and Pollux is immortal, because Castor is the son of Tyndareus and Pollux is the son of Zeus. Amazingly, even this doesn't cause a crack in their relationship. Never once in any source is there mentioned a thing about Castor being jealous of his brother's immortality.
When Castor lies dying, Pollux actually begs Zeus to let him share his immortality with his brother. So, in the end, both brothers end up being half-immortal, either shining eternally as Gemini, or alternating days between Olympus and the Underworld. Though the Dioscuri were the patrons of many things—horsemen, travelers, sailors, athletes, warriors—what they seem to truly represent is the power of brotherly love.