Milhouse is probably Bart's best friend.
He has a crush on Lisa. In the future he will almost get married to Lisa and in a different future he gets upset when he hears that Lisa is getting married. He will be Homer's boss at the Power Plant and when he hears Lisa will be getting married he says to Homer,"I think I'll right your evaluation now."
He speacks perfect Italian because when he would visit his Italian grandmother and speak English she would chase him around the countryside.
Milhouse is what some would call a troubled child. He sees a therapist. (Dr. Sally Wexler) He gets beat up at school. In his words his mom thinks he's cool.
He often tries to be cool. In one episode Bart goes missing and he "takes over" for Bart. Many people commented that it was pathetic.
Bart got Milhouse's picture on America's Most Wanted. The cops chase him.
His first girlfriend was Samantha Stanky. All they did was kiss. She was trasfered to an all girls school.
His second girlfriend was Greta Wolfcastle. She is Ranier Wolfcastle's daughter. When Bart broke up with Greta she was crushed. She started dating Milhouse to get back at Bart. When they both chased Greta to Canada and started fighting over her she promised that she would never date either of them.
Quotes: "I'm not a nerd Bart. Nerd's are smart."
"Everything's coming up Milhouse."
"Look out Itchy, he's Irish" (on Chester J. Lampwick's original Itchy cartoon.)
(shouting) "Spring Break! Yeah!" (after funny looks) "Well, when are we going to get rowdy?"
"A cup holder! Bart, we've gotta stop and get a cup."
"Hey, kid: stop wearing you're backpack over one shoulder. We invented that, copycats."

Milhouse went to the Flanderses beach house for the summer
with the Simpsons. On the car ride he sat in the baby seat.
Bart says something along the lines of,"Milhouse is squirming.
I'm gonna tighten his seat belt." He does just that.