Dating Robbie
It is revealed in The Hand That Rocks the Mabel that Wendy has broken up with way too many guys. It could be part of her lumberjack genes. She probably considered Robbie to be just another guy, explaining their relatively short relationship (only 8 episodes) and subsequent breakup. It's very hard to tell what happens to Wendy after said breakup, because she got hardly any screen time between then and the end of the season.
Not much character development or screen time
Out of all the 5 protagonists, Wendy has by far the least amount of screen time. In fact, every other protagonist appears in every episode of season 1 (except for Soos, who does not appear in Irrational Treasure, but is mentioned by Stan). Wendy only appears in 16 of the 20 episodes. But don't worry, she'll have some more development in season 2.
This was briefly mentioned in Into the Bunker, when Wendy said "I've been wanting to go adventuring with you guys." There's still debate over how important she actually is, but I think she is just as important as Dipper, Mabel, Stan or Soos.
Other
When someone is playfully showing affection in Gravity Falls, the most common action they will take is lightly punching the other person's arm. Wendy is the character who does this the most often, and even more interesting, the person she does this to the most is Dipper.
