I don't normally care about PLOT in a capcom fighter but wow FrankenNash is such an abysmally terrible character. Like it's not even funny, they deliberately sabotaged the art style of the SF universe for some desperate appeal to nostalgia by making this horrible edgy character.
I disagree (though, to be fair, I am a huge Charlie fan, so blinders might be on). I find this direction to be an interesting and depressing turn for him. He was the guy who put it all on the line for justice (his SFalpha nickname was literally "Cool Justice") and he got betrayed by the upper brass until his "death". He lost everything in his quest and somehow has been given a second chance at a terrible price.
He's been revived by the Secret Society/Gill and we don't know the extent of manipulation they've done. For one, he's got a Third Eye Opal, only Gill and Urien have been seen with those, not even Gill's beloved right-hand-woman, Kolin has one, so that's gotta be significant. They're not above twisting people in their preferred direction, but don't do Bison-level brainwashing. Anyways, he in alpha was serious about Bison, but he allowed himself an amount of a sense of humour, lovable arrogance, love of scotch and friends, (heck, even though it was part of infiltration, he and Chun Li posed as a couple for an extended amount of time pre-alpha 3, something had to have blossomed there).
He back and he's totally focused on Bison and damn the consequences. His body represents his failure and betrayal and the sad truth that going against evil is going to leave you disfigured physically and mentally. I personally could see him going in a Big Boss direction and declaring that the whole system stinks and needs to be burned to the ground and rebuilt.
I'm not happy his and Chun's quotes don't seem to show they know each other really well, but each of those tells something about him:
-vs Birdie: he may have vague memories of him being Shadaloo related so there' the +1 for hatred, but also, it shows he's completely lost his sense of humour and calm, analytical attitude
-vs Ryu: He doesn't know Gouken sealed the SNH in SF4, but also, with his search for Bison, all he kept hearing about was Bison's obsession over Ryu due to the SNH having similar properties to Psycho power. So Charlie isn't impressed, Ryu didn't have any close ties to Shadaloo, he's disappointed, but will let him live.
Vs himself: He may not fully remember everything that's happened to him, thus, affected memory makes him easier to maniuplate
vs Cammy: Again, remembers her doll mode, Shadaloo connections make him unreasonable and focused on its utter destruction, shows his past sense of reasoning is gone or clouded fully by hatred or outright manipulation by Gill and his Illuminati/Secret Society
vs Chun Li: He's bluffing, in his intro he gives the warning, but with him having warned her he's renegging and giving her another chance. Personally I think they could have had something more intimate for the quote, but oh well. Anyways, he might remember a bit of something about her and thus is letting her live to fight another day.
Opinions differ, but at this stage, I think it's premature to call him "abysmally terrible" when we don't even have access to his story. In my opinion,
abysmally terrible goes to Ingrid:
Gothic lolita, Mary Sue that's a literal time travelling goddess, who holds the key/elements of psycho power yet doesn't explain anything, is so OP that Rose can't even get a read on her (yet, she can still read Bison at his most powerful), may also have telekinesis (she lifted Sagat's statue herself or with a tractor beam) and never really has any significant interactions or growth potential in her story. She just drops out of nowhere, changes the entirety of understanding of Psycho Power, messes with the upper ceiling of the SF world, and poofs out of time. That to me is terribly executed.