Ford knew what he was almost immediately after the transformation occurred. The signs were too many to ignore, and he knew them better than any man alive (of which he was no longer one). Denial could only go so far.
Knowing what he was was one thing; gathering up the courage to admit it, however, was another.
He didn't lie outright, but he told half-truths and didn't protest when they called him a ghost, and that was enough to prevent the others from realizing the truth initially.
It took several days and one very messy infodump before Ford confessed what he had known to be the case all along: he was a demon now, his greatest enemy become the essence of his very being.
Stan and the kids accepted this without hesitation, telling Ford that he was the same person he'd always been, that this wouldn't change anything between them.
Ford only wished he could believe them.
Ford had spent much of his life in pursuit of knowledge, and now that he was a demon, almost all that there was to know about the world lay at his very fingertips. All he had to do was want to know something, and more than likely the knowledge would be within his reach, only a thought away.
And there were a lot of things Ford wanted to know.
Some of these searches grew... messy, as Ford couldn't always keep control when he was poring through the vast stores of information within his mind, but that usually didn't stop him. What cost was a few minor changes to the physical world when Ford could learn the answers to so many of the questions that had been plaguing him for decades on end?
But over time, the novelty wore off, and Ford began to miss learning things the old-fashioned way. He lost interest in reading books when he would know the ending before he even picked it up, and the experiments and exploration that had once taken up so much of his time were just a relatively inefficient means of acquiring knowledge. Omniscience gave him answers, but it took as much as it gave.
And, of course, as years became decades became centuries, Ford learned perhaps the most important bit of knowledge he could have: knowledge isn't everything. He could know everything there was to know about Dipper and Mabel, but that wouldn't change the fact that he hadn't been there for the first twelve years of their life, nor would it grant him the relationship with them he so deeply desired. What mattered was being there for them now, doing what he could for them. Facts were nice- facts were wonderful, facts were amazing- but mere facts could never replace the people in his life, nor would Ford really want them to.
(Though Mabel was impressed that he could guess her favorite movie on the first try.)
When a summoner first asked Ford what his name was, he spat out the word Pollux before fully weighing his options.
It wasn't a perfect choice; there were parts of Pollux's mythology that didn't correspond to his life, even in the vaguest of senses. Perhaps, had he thought about it for longer, a more fitting name would have occurred to him. But Ford didn't dislike the name, and it certainly did fit in some ways- he was a twin, and he was a Gemini, and... well, maybe that was enough. It had a good ring to it, anyway.
Stan was less than impressed when informed that this would make him Castor ("What, you mean like castor oil?"), but it did help for Stan to have a go-to alias to give when he got in too deep... and Stan did have a habit of getting in too deep.
It was only a few short years after the Transcendence (Ford was still, privately, bitter that the name Weirdmageddon hadn't caught on outside of Gravity Falls), and Stan was bleeding out in the middle of a summoning circle, and Ford was there but it was almost too late- there was only one exchange that would save Stan's life now, and Ford didn't want to do it, didn't want to carry the burden of owning his brother's soul- owning souls was Bill's modus operandi, not Ford's, never Ford's, at least not before this-
But he owed it to Stan to at least bring up the possibility. Trading one's immortal soul for a couple more decades spent in this lifetime... it was obvious who was getting the better deal on that trade, and Stan would be able to tell, Stan who only took one-sided deals when he was the one who would be making out like a bandit-
Part of Ford thought that Stan would turn down the offer.
Part of Ford wanted Stan to turn down the offer.
But Stan just smiled and agreed and made some comment about his soul not being worth much that Ford fervently hoped was meant as a joke (but he knew better- there was no humor in this situation, no levity in the blue-black-smorple colors swirling around Stan's dying body).
As Ford reached out to his brother, he remembered how in Greek mythology Pollux, the immortal twin, made a great sacrifice to stop the death of his mortal brother.
Five fingers grasped six, and Ford decided that his hastily-chosen demon name fit well after all.
There are several versions of the Castor and Pollux myth- the details of their birth are shrouded in uncertainty, as are the details surrounding Castor's death.
Pollux successfully offered to split his immortality with his twin brother, that much is generally consistent, but the exact details of the arrangement vary from account to account. In some, Castor and Pollux would trade places regularly, one always in Olympus and the other always in Hades, the two never to be together again. In others, Castor would stay in the underworld, and Pollux would be permitted to visit every other day, spending half of his time alive but unable to see his beloved brother. In both cases, the outcome, while better than the alternative, has significant shortcomings that the narrative usually glosses over; in both cases, the brothers spend at least as much time apart as they do together, and Pollux's immortality hurts their cause as much as it helps.
As Ford hovered unseen over the grave of yet another reincarnation of his twin brother, he silently cursed the narrative, cursed the idea that what became of Castor and Pollux counted as a happy ending.