I'd like to guess Professor Horace Slughorn
I've got some reasoning.
Never was guilty,
Despite overwhelming proof,
He is sort of guilty of aiding Voldemort in learning how to make a horcrux. But he has never gone over to the dark side, and has morals. So in this way, he doesn't truly have guilt.
A Half blooded friend,
Of justice and truth,
It is unknown for sure what Slughorn's status is, but he is either pure blood or half blood. He is a friend of justice and truth in several ways: one he is a personal friend of A. Dumbledore, and two, he maintained his morals despite his ambition and refuses to join the death eaters, three, he helps harry, the figurehead of justice and truth.
Not known to their friends,
Who get angry and shun,
He befriends those he thinks will make it big, but shuns others. He also keeps secrets about his past with Voldie from everyone, even his friends. This secret makes Dumbledore and Harry upset because they need it.
Larger than life,
In more ways than one.
He's chubby and overindulgent. He also prides himself on having tons of friends in high places, thus making him important (and larger than life) by association.