Robbie and Dipper are arch-rivals, primarily driven by both wanting to win Wendy's affection. While Robbie views his relationship with his crush as a victory for himself, he fears Dipper will appeal to her. Robbie likes to make fun of Dipper, is very condescending in his remarks, and has even resorted to attempts at physical violence to prove his point. Dipper, on the other hand, is angered and annoyed by Robbie. Even though they are initially more vocal about their dislike of one another, they eventually agree not to brawl in front of Wendy, due to fear that they will both lose her if she sees them fight.
During the events of "The Love God", Robbie blames Dipper for Wendy dumping him and announces a hatred towards every member of the Pines family for "ruining his life". By the end of the episode though, it's unknown if Robbie and Dipper still hate each other since they have both given up on trying to win Wendy's affection.
In "Weirdmageddon Part 3: Take Back The Falls", Robbie is seen playfully chanting "one of us" after Dipper and Mabel blow out the candles on their cake and his signature is seen alongside many other signatures on a note that was given to Dipper by Wendy as a parting gift so that Dipper can remember them all. This indicates Robbie and Dipper have reached a truce after recent events.