Ford is Stan's twin brother. Inseparable as children, Stan was protective of his physically weaker twin, standing up to bullies for him and being wholeheartedly accepting of Ford's six fingers, even incorporating them into their personal salute (the "high six"). He seemed proud of his brother's brilliance at first, happily joining him in the celebratory picture when Ford won the Science Fair and shamelessly copying off of him at school. When Ford's intelligence began to provoke negative comparisons and threatened to end their plans of sailing the world together, Stan began to feel threatened, which led to teasing and belittling on his part; these emotions culminated in Stan venting his anger and fury at losing his brother on the infinite motion machine, unintentionally causing it to stop, destroying Ford's college plans. This caused Ford's refusal to intervene on his behalf when their father disowns him.
While Stan would hold a grudge about this for many years to come, he does not forget his brother. He calls Ford at least once, though Stan hangs up without saying a word, and kept a picture of them at boxing practice taped to his car's shade. He is also quick to seek Ford out once his brother sent him a postcard requesting his presence in Gravity Falls. Believing he'd been called to make amends, Stan lashes out in anger when Ford reveals he intends to make his brother go as far away from him as possible, which results in the fight that sends Ford through the portal. Stan is distraught at this turn of events, spending sleepless nights with Ford's journal and glasses in hand
Issues between them notwithstanding, Stan spent the next thirty years relentlessly trying to figure out how to reopen the portal and bring Ford back. When Ford does return, Stan is eager to welcome him back amicably, and is taken aback when Ford responds with a punch to the face instead. Stan quickly turns to resenting him when Ford refuses to thank him for bringing him back, mostly ignoring him and becoming jealous when the twins display any kind of admiration for his brother.
Stan's issues with his brother come to a head during Weirdmageddon, with Stan becoming increasingly resentful of the survivor group's focus on recovering Ford, as he interprets this as undue admiration towards Ford rather than a way of discovering Bill Cipher's weakness. In the Fearamid, Stan is reluctant to go along with Ford's plan, only agreeing to join the Zodiac Wheel after Ford thanks him. Ford's petty correction of Stan's grammar gets Stan agitated, leading to him attacking Ford, which disbands the wheel and causes it to fail.
While imprisoned by Bill, Stan and Ford are finally united in their distress over Dipper and Mabel. They reflect on their failings and what went wrong with the bond they used to share; while they discuss this, they come up with the plan to lure Bill into Stan's mind. While posing as Ford for the deal with Bill, Stan includes that Bill let his brother and the kids go as a condition to let him in.
Once Stan's memory is wiped and Bill is defeated, the now-amnesiac Stan does not react to his brother hugging him and calling him a hero, as he doesn't know who either of them is.
After his memory is fully recovered, Stan is deeply moved when Ford asks him to come along on his hunt for anomalies, as it represents fulfilling their childhood dream of sailing the world together. He seems to welcome Ford's comfort when he is sad after Dipper and Mabel leave, and they are later seen battling a giant squid together and sharing a laugh once it leaves, their bond clearly restored.