When he was a kid, Mario's father left to fight in World War Two, leaving him to be the man of the house while his mother worked two factory jobs. He moved to a creepy house in Franklin village and discovered a trapdoor in the basement that lead to a tunnel filled with zombies. He escaped but accidentally forgot about his friend Ivy, who was turned undead by the zombies. She also swore revenge.
This experience left him left him so distraught that he started having nightmares about zombies. He moved to Alaska, hoping they would stop but they didn't. He moved back to Franklin Village to sort things out, and his son Sal moved his family there to take care of him.
After a kid named Trevor moved in next door, he started to think he and his family are zombies, which led his grandson Kenny and his friend Alec to believe the same. He started wandering the graveyard late at night, which led his Kenny to believe he was a zombie. He had a run in with some zombies but they let him go, as he was now too skinny for them to eat. On Halloween night, Ivy escaped from the tunnel and attacked Kenny, believing he was Mo.
After showing up and correcting her, she was disappointed to that he was "too old" and went back into the tunnel. He then went to bed, and thus did not see the other zombie escape and attack Kenny, nor did he see Trevor and his his family reveal themselves to be vampires, so it is unknown if they also planned to go after him and the rest of the family.
As a little kid, Mo was into comic book heroes, but was forced to grow up when his father went off to war. As an adult, he tells zombie stories to his grandson and is left frazzled after his experience with the zombies as a kid, having constant nightmares.
He has thick wavy white hair, and his skin is tight on his narrow face and almost as white as his hair.