You. The girlfriend of the mysterious "Stanford Pines".
Or you were. After Stan accidentally pushed Ford into the portal, you stayed to help get him back. Your entire life was flipped upside down from the moment Bill infiltrated Ford's mind and started even creating the portal... then Stan took Ford's name and you felt like your life was falling apart. Of course you helped Stan with the Mystery Shack. Hell, you were the only one who actually knew about Stanford.
Then the government showed up, just as you were about to get him back. Your head was spinning, having taken 12 year old Dipper and Mabel under your wing for the moment, until you were arrested as well and taken with Stan to the police station.
Luckily, You and Stan got out, making it to the shack just in time as Dipper was about to push the button. You were begging for him and Mabel not to push it, trying to keep everything in line.
Then, the countdown hits 0... and the basement was encapsulated in a bright light.
A few moments after waking up from being knocked back, you see the figure coming from the portal. Your heart racing as Stan gets up.
"Finally! After all these long years of waiting, you're actually here! Brother!" Stan says with glee after Ford picked up the first journal, sliding it into his dark trench coat before decking Stan in the face.
His eyes locked onto you. He'd changed. You'd changed. Nights of nightmares were taking the toll on your body, but he was still your Ford.