Dipper considers Mabel his "Closest Ally and Best Friend".Being twins, they have been close throughout their entire lives. Despite getting on each other's nerves, contrasting personalities, and frequent sibling bickering, the two love each other regardless. They often engage in various recreational activities and enjoy having fun with each other, and they constantly investigate their summer home's mysterious secrets side by side. Dipper and Mabel give one another advice and are willing to sacrifice much for the others' sake, albeit not always initially, and have risked their lives more than once to help each other. However in "Not What He Seems", it's hinted that Dipper's relationship with Mabel may have been broken, due to her trusting Stan over him but in "A Tale of Two Stans", Dipper held no grudge towards her for trusting Stan but Mabel is left unsettled about their future relationship due to Stan and Ford's dysfunctional relationship. He does, however, hint in "Dungeons, Dungeons, and More Dungeons" that he may still be carrying some ill-feelings after Mabel and Grunkle Stan's numerous taunts at his interests, though they were never meant in bad faith.
In "Dipper and Mabel vs. the Future", Mabel overhears Dipper accepting Ford's offer to become his apprentice and is so terrified and upset at the thought of her and Dipper not growing up together that she runs off crying into the forest. She does not wish to see summer end and have to leave Gravity Falls, leading to her inadvertently handing the portal's rift to Bill Cipher. However, in "Weirdmageddon 2: Escape From Reality", Dipper convinces Mabel to leave her perfect prison bubble by promising her that he won't take Ford's apprenticeship and will always be there for her, having realized they can get through anything together, leading the twins to reconcile.