Rusty (played by Jordan Christopher Michael) is the son of Cindy, Danny's girlfriend, in season four. He has a reputation for being a troublemaker and a prankster.
History
In the episode "Terror in Tanner Town", he spends the day with the Tanners, and wreaks havoc throughout the household, including tucking the tablecloth into Danny's pants, punching a hole in the milk carton, giving D.J. and Stephanie trick gum that turns their teeth black, locking Jesse, Becky, Stephanie, and D.J in their rooms by tying a rope between both rooms' doorknobs, taping over Joey's comedy video with a clip of himself on his own comedy show (which he calls "Rustyvision"), making Danny fall in a mud puddle while playing football with him, feeding Comet a rump roast while D.J. and Stephanie are bathing him, getting Michelle’s eye painted black from looking into a kaleidoscope (although that was meant for Stephanie), and putting green hair dye in Danny’s shampoo.
It’s learned that the motivation of the pranks is to turn Danny away from Cindy, as Rusty hopes his parents will get back together. At the end of the episode, as he's enjoying a root beer float, D.J. and Stephanie turn the tables on him, as he falls victim to one of their pranks, by spraying him with whipped cream.
In “Secret Admirer”, when Cindy and Rusty come over to the Tanner family barbecue with Cindy's semi-homemade apple pie, it's discovered that Rusty ate a piece while they were on the way there. But that's only the beginning.
He causes a major commotion with a love letter he types (as he is inspired and intrigued by D.J.’s crush on Ricky, the paper boy). He intends for her to receive it (as well as make her think that it came from Ricky). However, he forgot to write “Dear D.J.” at the top of the letter (or address it to anyone else), and simply signed it “Hot for You”. As a result, it got around to everyone in the Tanner household from someone else, which eventually led to pandemonium until Jesse got everyone calmed down. When he said he wanted to know who wrote the letter, Michelle says she knows who it is and rats Rusty out (after Jesse asks her who it was). Rusty takes off, with everyone else chasing after him (except for D.J., who stays behind when Ricky arrives).
Rusty appears in only two more episodes, including: "Happy New Year" (in which Stephanie actually kisses him, after he keeps saying he will, and he turns out to have been lying) and "Stephanie Plays the Field", in which he’s on Danny’s Little League Baseball team. He comes up with the name for the Tanner Twister.