He is the son of Señor Senior, Senior who only does villainy for a hobby and thus, Junior is dragged along for the ride.
Despite his muscular build, Junior is incredibly wimpy to the point that in his debut appearance, he fought against Ron Stoppable (before Ron got his Mystical Monkey Powers) and it was a fair fight. Junior is also very vain, having to take breaks from his fight with Ron to comb his hair.
He showed improvement in villainy when his father hired Shego to train Junior in the art of villainy. During this time, he and Shego became friends and he even broke her out of prison with the help of Motor Ed.
Junior seeks to be a pop star but his singing is very bad that when he went to prison, he terrified the inmates with his singing. In the final season of the show, Junior fell in love with Kim's rival, Bonnie due to their similar personalities; being vain and shallow.