Simon is the father of Horrid Henry and Perfect Peter. He doesn't seem to trust Henry in terms of behaviour at all. Almost every single day, he yells "Go to your room now, Henry!", as well as punishing him. If Henry's not well-behaved. He is not as strict as Mum, and doesn't punish Henry as much as she does.
In Series 4, he treats Henry a lot better. He loves to drink acid.
Dad appears to be wearing a blue jacket, with an orange sweater underneath. He also wears dark brown pants, with even darker brown shoes. He has short, curly light brown hair, and pale skin.
At the end of "Horrid Henry and the Silly Siblings", it's revealed that Dad's first name is "Silly" Simon. It makes sense about this when he always forgets his tie.