Growing tired of his son's weak nature, Filbrick forced a young Stan Pines into boxing lessons, hoping it would help him build a stronger character. Though Stan believed it was done simply because his father disliked him, he later grew to appreciate the lasting benefits, later repeating the same process with his great-nephew, Dipper Pines.
He also had Ford, his other son, take boxing lessons, seen in the background of Stan's flashback (reading a book, covering his face) as well as in the picture in Stan's car.
When Stan accidentally damaged Ford's perpetual motion machine, ruining his twin's chances of getting into his dream college, Filbrick believed he had done on purpose, and angrily disowned Stan for ruining the family's chances of being rich. He threw Stan out of the house, telling him he could only come back when he'd earned the kind of fortune he'd cost them.
During Stan's recollection of his childhood in "A Tale of Two Stans", he described his father as "tough as a cinderblock and not easily impressed", though he later admitted to feeling the latter following his son Ford's possible acceptance to West Coast Tech. However, after hearing Stan had destroyed his chances at admittance, he disowns his son and barred him from the family until he could return with the millions he cost Ford.
From what little we hear of him from Stan, it's implied that he was tough, but fair; only pushing Stan so he would be the best he could be. He also seems to be a man of few words, as in all his time onscreen, his only reaction to his surroundings is a simple nod of the head.
It's revealed in "A Tale of Two Stans" that he's very honest and generally unimpressed with everything, the exception being when Ford Pines was qualified for West Coast Tech. He seems serious about his children making a lot of money, as shown when he kicked Stan out of the family due to him ruining Ford's project, causing them to lose a lot of money, as it was promised that Ford could become a millionaire. He is also shown to be quite naive, as he immediately assumed that Stan had ruined Ford's project on purpose and refused to believe that it was just an accident.
- 119. "Dreamscaperers" (flashback, no lines)
"I'm not impressed."
" You did what, you knucklehead? "