Bowser Jr. first appeared in Super Mario Sunshine as the major antagonist, and has since appeared in almost all Mario sports games since, as well as most main series Mario games.
In SSB4, Bowser Jr. would be a fairly small character, as well as being fairly light. However, he would make up for it with his powerful attacks. He would have average running speed, but I can't imagine him being particularly mobile in the air. He could use his magic paintbrush from Super Mario Sunshine for attacks.
The possibilities for his Final Smash are almost endless. He could transform into Shadow Mario, he has used Bullet Bill cannons in the past, so this is also a possibility. He has his Sonic Roar from Mario Strikers Charged. And finally, he has also been shown to have a large amount of influence over the Koopa Troop, so he could be able to bring them in.
