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.
These two brothers are Castor and Pollux's cousins. When the Dioscuri kidnap the lovely Phoebe and Hilaeira, who are engaged to Lynceus and Idas, a rivalry begins that will relieve all four men of their mortal lives. When you think about it, Lynceus and Idas have pretty good reason to be ticked off at their cousins. How would you feel if your cousin rode up, stole your fiancé, and forced him or her into marriage? Yeah, it would suck, right? Some tellings of the tale even explicitly say that Castor and Pollux rape Phoebe and Hilaeiria, which would most certainly give Lynceus and Idas good reason to hate their cousins.
Even in the conflict over the cattle that leads to the death of both pairs of brothers, the Dioscuri are far from innocent. For starters, they all stole the cattle to begin with. After that, though, Idas wins all the cattle for him and his brother by betting that they can finish their steak dinners faster than the Dioscuri. Sure, he's being a little sneaky, because the Dioscuri aren't totally aware that he's a particularly fast eater. But still, a bet's a bet, right? It seems like Lynceus and Idas have some right to be furious with Castor and Pollux they try to steal the cattle back later on.
What's interesting is that even though Lynceus and Idas might seem to be on the right side of this rivalry, they're shipped straight to the Underworld after they die. The Dioscuri, however, are allowed by Zeus to share Pollux's immortality and go on being half-immortal forever. Wow, guess it really pays to be related to the king of the Gods, right? In their relationship with Lynceus and Idas, we see that the Dioscuri definitely aren't as perfect as they might seem when you first meet them.
This lovely lady was the queen of Sparta and the mother of Castor and Pollux, who were said to have hatched from a big old egg that she laid. Why would a human queen lay an egg, you ask? Because she mated with Zeus while he was in the form of a swan, of course. Later that night, she also slept with her husband King Tyndareus, which is why both of the Dioscuri weren't immortal. Go-figure stories like this pop up all the time in Greek mythology. Leda is one of many mothers of demi-god heroes who was said to have mated with her husband and a god in the same night, therefore giving their son half-god status. Another great example is Aethra, mother of Theseus.