Castor

As you may have guessed by drawing similarities to Persona 3 (or already known), Polydeuces is Akihiko's original Persona. Just like the two horse patrons, Shinjiro and Akihiko are practically brothers who were always together in the past. Akihiko is a boxer just like Polydeuces. Shinjiro is not a horse trainer, but his Persona is depicted riding on one. Castor being mortal and thus causing Polydeuces despair parallels Shinjiro's ultimate fate and grief experienced by Akihiko. Their designs also are symbolic of the myth. Polydeuces is white in color (immortal), whereas Castor looks like a darker version of him with a spear through his chest (death). The spear through Castor could also be a representation of the pain and self-hatred Shinjiro feels.
In the Persona 3 Official Design Works book, the designer says Castor's model was to be Polydeuces' older brother. His riding horse and military affairs are that of an excellent brave warrior of Sparta. He also notes that Polydeuces lost Castor right in front of his eyes when he was impaled and killed in action.
RESOURCE:
Castor & Pollux @ Myths Encyclopedia
b a c k . c l e a r . f o r w a r d Paint it Black is © Samantha, however Shinjiro and Persona 3 are © Atlus and other rightful owners. Paint it Black is a part of AFTER-DEATH.ORG.