He is the father of Fishy Boopkins. He is a massive sea monster sporting a blue suit, a red tie (both similar to that of Octodad's), and also possessing two enormous tentacles that can destroy just about anyone or anything in his path.
Fishy Boopkins often calls upon him in desperate times by shouting "Daddy! He's being mean to me..."
Due to the blue suit and yellow-orange tentacles, it is presumed that his model is actually Octodad with a Cthulhu head edited on.
In terms of personality, Boopkin's dad will always be hostile towards those who torture his son. However, he can also be helpful, such as sending people to where they need to go.
In Mario and the Experiment, Fishy summons his dad to take down Godzilla. However, he proves to be a much more difficult opponent than what he normally faces. Thanks to Mario unintentionally launching himself out from the cannon and grappling Godzilla, Mr. Boopkins wins.