Negi must have been bored, or just badly wanted to make a joke character. The ridiculous over the top panels with their "real" dad, he was like a JoJo villain.
On a side note, it's less creepy after I thought about the age of the quints, and realized this guy was probably 30ish when their Mom got hired as a teacher. It's still hard not to picture him as is though, before they were born, since he's never been shown any other way.