Prior to the fifteenth episode of the second season, The Last Mabelcorn, Billford was an obscure rarepair formed on the foundation that Ford and Bill definitely had a history of some sorts in the past.
The episode delved deeper into the past relationship of Ford and Bill, revealing they had once been friends and partners. Ford and Bill worked together on the portal (alongside McGucket who was left in the dark regarding Bill's presence) and Bill was allowed to move in and out of Ford's mind as he pleased. Ford clearly trusted Bill, considered him a friend and idolized him, creating an entire shrine to him in his study and modeling parts (such as the windows) of what is now the Mystery Shack after Bill.
Because of their past friendship and Ford's obsession with Bill, they have been interpreted to be closer than the show let on or didn't outright say. The notion that Bill and Ford were in a romantic relationship at some point was not outlandish and had plenty of ambiguous canon evidence, and thus the Billford community grew and converted many shippers overnight.
Billford is commonly interpreted as the two being exes, and either half is still in love with the other or bitter about their relationship's end all these years later.
Fanworks also depict younger Ford having a crush on Bill (such as blushing around him) and vice versa, whereas other works portray them in a loving, affectionate relationship (despite Bill's true plans). Billford shippers also tend to believe Ford was extremely distraught about Bill lying and betraying him, thus taking their breakup very hard -- afterwards, Ford swears revenge against Bill and promises to defeat him, whereas Bill would rather Ford join him (and possibly get back together).
Canon Evidence EditAlthough Billford is not canon, plenty of interactions between the two can be interpreted as romantic or flirtatious.
- Bill appears in Ford's dream and messes with him both physically and emotionally; some interpret this as flirtatious.
- "Just let me into your mind, Stanford!" "Please, call me... a friend." Bill and Ford shake hands.
- Ford's study resembles a worship shrine to Bill, emphasizing his obsession with Bill.
- "Hiya smart guy! [...] And your name's Stanford Pines, the man who changed the world! But I'm getting ahead of ourselves! Let's relax! Care for a game of inter-dimensional chess? Have a cup of tea."
- Bill refers to Ford with nicknames such as 'Sixer', 'Six Fingers', 'IQ', and 'Fordsy'.
- Bill calls Ford cute when Ford insinuates he can stop him.
- "I've been waiting an ETERNITY to have a chat face-to-face. Everyone, this Armageddon wouldn't be possible without help from our friend here. Give him a six-fingered hand! [...] Aw, don't look so sour, Fordsy, it's not too late to join me! With that extra finger, you'd fit right in with my freaks!"
- Bill solidifies Ford into gold and uses him as a backscratcher.
- Bill turns every single person in Gravity Falls into stone and uses them to build a throne, except for Ford, who he specifically keeps at his side at all times.