Dirk should have been Jake there, oops. And I know Jr. and Jake are action explorer buddies with the worst fashion sense and Roxy is one of the few people Jr. can talk to about stuff relating to things from his world without explaining. I'm also 99% sure Jr. would refuse to call Dirk Bro, since he doesn't throw familial terms like that around lightly.

Possibly sparring buddies, since Jr.'s a swordsman and inhumanly fast/strong?