Mr. Fierro was born to a potter from Mexico City. His father build up a company based on his skill, taking the time to insure that every peace was flawless. However he cared more about making a profit and their relationship was strained because of this. He would industrialize the company when he took it over.
He was married to an unnamed woman when Loki seduced him in the masquerade of a red haired woman. Nine months later his lover returned with a child. She told him that she was really a Norse god and opened him up to the Nine Worlds. He and his wife were never the same after that and they resented Alex for that. He would berate her for her heritage and being genderfluid until Alex was kicked out and forced to live on the streets.
Alex mentions her father when explaining what life was like for her. When Alex and Magnus Chase are trying to break an illusion during a bowling game against giants, Magnus sees a memory of Alex's father yelling at her to get out and stay out.
Magnus dreams of seeing Alex being thrown out of her house by her father, who angrily calls her out for not being normal and that his money will be going to his real children and tells her that she could have been as skilled with pottery as her grandfather before slamming the door on Alex.
Mr. Fierro is described as a man with dark hair and a skinny build. He often dresses in business attire or tennis clothes.
Mr. Fierro is not very faithful, as he had cheated on his wife with another woman. He was also cruel as he berated and disowned his own child for an act he and his lover committed.
Mr. Fierro had a strained relationship with his father, who accepted Alex being genderfluid.
Despite being married to her, he cheated on her with a woman who later revealed that she was really Loki in disguise.
Mr. Fierro hated his child for being gender fluid and a demigod. He would berate and abuse her for what her biological mother did to him.
While Mr. Fierro was at first attracted to the trickster while he was disguised as a woman to the point of cheating on his wife, he was shocked and revolted by the revelation that she was really a Norse deity and told them about the Nine Worlds. The shock would turn to hatred which he would take out on his child.
- Mr. Fierro is the only known mortal within any series to have a demigod child through an affair, when normally it is the god who has a child through this relationship.
- He plays tennis.
- In The Ship of the Dead he mentioned he has other children besides Alex.