Santa's Little Helper
Bart first met SLH, a Greyhound at the Springfield Downs after SLH was rejected by his owner for losing too many times. Since then they've been really good friends. SLH has been forced to go to a dog training school and even be a guard dog for evil tyrant Mr. Burns. Burns once even stolen all SLH's puppies. Bart even rescued SLH after accidently giving him away. But after all of this, Bart and Santa's Little Helper remain the best of friends.
Milhouse Van Houten
Bart's other friend Milhouse is known as a nerd, poindexter, specky, four-eyes amongst other things, but he's stayed friends with Bart for a long time. They once both ran hyper through Springfield after trying an All-Syrup squishee and ended up joinng Flanders' scouts group. They also ran the Android's Dungeon comic shop. They have fought before, and both sometimes get bullied by Jimbo, Dolph, Kearney and Nelson. They have still been friends, and will be in the future.
Like all brothers and sisters, Lisa and Bart often have arguments on who's smarter, what to watch on TV etc, but usually end becoming friends in the end. They have helped each other get through life, since they were little- Lisa has helped Bart get his soul back and helped him with schoolwork, and Bart helped Lisa pass a Military test by cheering her on and also buying her a rare record she couldn't afford made by her old friend Bleeding Gums Murphy. They have also foiled many plans by the evil Sideshow Bob.