Billy - ESFP (Fun-loving, friendly, and a natural performer. My little bro is ESFP, and him and Billy act alike in that sense)
Mandy - INTJ (Calculating, rational, and deadpan. I've interacted with enough INTJs to figure this out.)
Grim - INFP (Gullible, protective, and sensitive. I am all of these things, and we share similar behavior [I'm like my husband, yay!!])
Mindy - unsure (I've only seen her in one epsiode)
Jeff - INFP (Caring, sensitive, people-oriented. He wants to make his "father" happy, and is willing to compromise a lot. I identify with Jeff only second to Grim)
Eris - unsure (I have never watched an episode involving her)
Pudn - unsure (He doesn't seem to have much of a personality to me)
Irwin - INTP (Easygoing, logical, quiet. He's essentially a logical Jeff, just as INTP is basically logical INFP)
Nergal - ENTJ (Innovative, energetic, straightforward. Dad is about as ENTJ as you can get, and Nergal acts just like Dad.)
Junior - ISFP (Sensitive, tolerant, gentle. I was an ISFP when I was younger, and was just a louder Junior [I had hearing problems])