Pollux is the son of Dionysus and the twin brother of Castor.
Percy Jackson and the Olympians
The Battle of the Labyrinth
Pollux and Castor are both seventeen. Pollux sees his twin brother, Castor, die in battle. At Castor's funeral, Pollux tried to speak for his brother, but couldn't choke out the words due to his grief. He lit the funeral pyre, and sat back down mornfully, trying not to cry, Dionysus (their father) is seen following him out to make sure he's alright.
The Last Olympian
In The Last Olympian, Pollux is the only member of his cabin. He fights bravely in battle, calms the other demigods down when they are worried and anxious about the spy amongst them, and is shown to be a good warrior, and have common sense.
He breaks his arm in battle, and Percy insists he not get back into battle, but doesn't tell him why. He tells Pollux it's a "personal favor." Dionysus specifically requested that Percy keep his son safe in a dream Percy had, and Percy didn't want to embarrass him by revealing it. This possibly implicates that Dionysus had been watching over his son after the death of Castor, and perhaps Pollux hadn't dealt with Castor's death well.
Personality
Pollux is level-headed, and a good warrior. He is not very close to Percy, Annabeth, and Grover, but is very brave and protective of his brother, his best friend. He was willing to fight with his left hand, though he is only practiced in his right, which would be suicide. This shows how much he loved his brother, being willing to die for him.
Appearance
Pollux has a round face like his father, along with his father's dark eyes and curly blond hair (though Dionysus has black hair). He is very good-looking and has an athletic look to him. He and his brother, Castor are/were identical.
Relationship with Castor
It is shown that they were very close, and that they loved each other very much. Pollux was brokenhearted and crestfallen at his brother's death, and this helped his fury against Kronos and his monsters, so he had no doubts about him fighting for his brother's memory. Also he was prepared to charge suicidally into battle against the titans to honor his brother's memory and avenge him.
Dionysus, Pollux's father
Abilities
Pollux can control grapes and strawberry plants, using his powers he can grow vines and use them for various things like grapples or restraints, just like his father, except his powers are not restricted. He and his father are very much alike when it comes to looks, some aspects of their personalities and their abilities.