Daddy Dearest is the true mastermind
Spreading a deadly Virus and causing the destruction of New York City just to kill your daughters Boyfriend is probably something Daddy Dearest would do this is the same man who trapped someone in a dating sim after all.- Jossed, I find it hard to believe that he would back any plan that would lead to Girlfriend being affected by it too.
Trallface is the strongest of all Trollges
He can pull a All Your Powers Combined on BF and seemingly warp his background in a really chaotic way. Also, he's a meme character, and as we all know, meme characters are overpowered yes or yes. The Trollface is not the original source of the T Cells, but he is the source of the other Trollges.
Trollge originated as a corrupted version of the Trollface. It wasn't until the Trollge meme itself began to spread that unique Trollges began existing.Also, remember JJ, or The Happy Clown, a Trollge specifically mentioned to have never come into contact with whatever caused the other Trollges' mutations. Perhaps JJ already had T Cells dormant in his body, and it was just his dark thoughts that triggered his mutation? What if all Trollfaces have T Cells, waiting to begin spreading from dark thoughts? If an update where we fight against the OG Trollface ever comes out, he will not look monstruous at all, at first
It will show him as the goofy prankster he usually is right before things have a turn for the worse and he turns into his Trollge form. Trollges are Undead entities
Trollges have been referred to being skeletal on multiple occasions (Smiler calling himself a skeleton monster, Legion talking about the feeling of water on his bones). In addition, it has been explicitly stated that Exhalated has been revived from the dead after drowning himself. And then there's the fact that the motto of the TCF is "What Is Dead, Must Stay Dead." It can also explain why Girlfriend's Beauty Is Never Tarnished: She still has her flesh. Trollge Girlfriend looks different than the other Trollge’s because she’s a demon.
Think about it the other Trollge’s were human before the infection and they look more skeletal in their Trollge form, while Girlfriend was always a demon and her Trollege form is significantly different from the rest. Boyfriend's black arm in "大好き" is a remnant of his Thanatophobia Sanity Slippage.
It's not clear if it was what it really happened, but it was possible Boyfriend might've almost become a Trollge due to his breakdown (for the song's first half he has the Slasher Smile, and through all of it he has the white skin). While he managed to recover, just like his Girlfriend, he is Locked into Strangeness because of it (although only with a black arm instead of having a muted palette because his transformation was never completed).