Star Swirl the Bearded is a character from My Little Pony: Friendship is Magic.
One of the most (if not the most) powerful unicorns alive, Star Swirl acted as both a mentor and father figure to Princess Celestia and Princess Luna, raising them and teaching all about magic and about ruling Equestria.
Star Swirl is a member of a team of legendary Pony heroes known as the Pillars of Equestria, who protected Equestria before the Mane Six were around. In one of his battles against the Pony of Shadows, Star Swirl was trapped in a pocket dimension alongside the rest of the Pillars.
Star Swirl became a much-revered figure around Equestria, especially by Twilight Sparkle, who deeply admires Star Swirl due to his wisdom and power.
Eventually, the Mane Six free Star Swirl from the pocket dimension. Reuniting with Celestia and Luna, he continues to protect Equestria and acts as a mentor not just to them, but to the Mane Six as well.