Mother - While she wasn't the best parent, she wasn't bad either. She did the best she could given that the whole family had more 'villainous' quirks. Life was hard for the family of four and both she and his father did the best they could with what they had right up until the end. 

She's where he gets the ability to blind his foes as well as his need for blood or raw meats.

Father - Ever firm and seldom speaking his father was a hard-working man who did everything he could to support his two sons. He saw little of his family given he worked two separate jobs. His quirk lends it's strength and resistance to Santiago. 

He was kind but stern and did his best to teach Santiago right from wrong right up until the end.

Brother - We do not speak of this.

  For the most part, his early years were normal enough. He was a year older than his younger brother who he had always adored. Protective of him, kind to him, he was the ideal older brother and his parents were proud and pleased with this. 

  With his brother's quirk manifesting first Santiago was a little worried when he hadn't figured out what his was right away, but fears were for nothing as he was simply a little bit of a late bloomer. His quirk manifested and he was itching to test it out. While his brother had had a sort of mutation in his quirk where he could paralyze with a gaze Santiago only knew of his ability to blind someone. He wasn't aware of the unfortunate drawback. 

  After the accident, Santiago was never the same. He had fallen into a deep depression and self-loathing right up until his twelfth birthday when he ran away from home. He's lived on the streets since unwilling to make friends with others and instead resorting to petty crimes to feed himself. He's often found in back alleyways or in abandoned buildings where he finds occasional shelter. 

 ...