He is a light blue marlin with a long nose and a wizard hat. He controls the weather in Rock Bottom. He hates Gary Gulper because he took his job and because he can predict the weather too. He lives in his home.
He makes it rain in Rock Bottom because of the weather station's machine that potentially could control the weather, which is Marlin's job. He then sends SpongeBob to sabotage Gary Gulper's weather-controlling device with his own magic wand, which SpongeBob uses to get past the door.