despiteeverything | THINGS AGAIN

Canonpoint: Post-True Pacifist ending, yes to both questions you're asked at that time.

Okay honestly at this point this thing is super out-of-date and idefk what I'm doing anymore (some sections more than others). Generally speaking, I'm willing to play with a lot of different things. What I'm listing here is primarily going to be for the version I use for the Frisk I play snowblindrpg but just take things here with a grain of salt. Empatheias Frisk has their own info post, with one exception:

My Frisk is not Flavor Text/Narrator Theory Compliant. This is universal.

Backstory - Frisk doesn't have any sort of defined backstory, so all I really have to go on is headcanon with implications from Frisk's behavior in-game. So here's what I sort of have mentally developed?

Frisk is an orphan. I do think that it would have happened when Frisk was rather young. Old enough for Frisk to remember what an actual loving family relationship is like - enough to recognize it in the way Toriel treated them. But young enough that it - by itself - is mostly an old hurt now.
Not sure how far I'm going down the abandoned quiche theory yet.
They didn't climb the mountain for happy reasons either (though probably less unhappy than Chara's)
This is another section that is prone to revision rn

Timelines - This could overall be interpreted SO MANY WAYS but to generally get the full grasp/scope of everything, the way I'm going to play Frisk is as such:

First runthrough was a Neutral Pacifist run because that's how Frisk do. After this comes the first major reset.
Frisk is so disoriented by this that Chara gets a hold and begins to influence actions, leading to No Mercy/Bad Time run. When Sans spares them though, Frisk finally manages to reassert control and spare him in return, leading to their death and the second major reset.
This run however, Frisk is prepared for Chara and able to act on the hint Flowey gave them at the end of Neutral Pacifist. This in turn allows for the True Pacifist run which leads to the True Pacifist ending.
If I wanted to do a more drawn out/complicated version, there could be other timelines in between Genocide and True Pacifist where Frisk either messes up or just gets tired and Chara gets another foot in the door. ~~I'm not sure how much I'm feeling the whole repeated mental battle for dominance thing though so talk to me if you're interested in this at all.~~ Still talk to me if interested, although moreso now because I don't want to trip onto the path of needless cruelty in backstory lmao.
I play Frisk as fully remembering each reset and reload. Flowey seemed to be able to do so when they were the one with the most Determination, so it would make sense that Frisk would be able to as well once their Determination overrode Flowey's.

Chara - this one is a bit tricky - what sort of relationship is there between Frisk and Chara, and how is Chara able to influence Frisk, and how does that affect things in the game and in the future?

Basically disregard almost everything I've ever written here previously god I wish I didn't work full time sometimes I wish I had more time to poke at this
In the simplified version of the timeline, the actions taken by Frisk and Chara are mostly separate, with one of them in charge or the other. There's not too much blurring identities. In the reset-heavy version things got a little dicey during some of those middle neutral runs.
Frisk feels Chara as more of a "pressure" than anything, a weight where Chara is trying to influence them in another direction. The pressure is heavier at some times than others. When Frisk doesn't have the energy to fight through it, or else is too distracted or startled to do so, that's when Chara's able to take over.
There also is some emotional feedback. But it's still more of a general feeling, with specifics being very difficult to pin down.
(Partially because of that) I don't think Frisk really fully hates Chara by the end of things. Especially after Asriel's explanation. Do they still think they should have stopped Chara? Absolutely. But hatred I don't think so.
THIS IS PROBABLY GOING TO BE SUBJECT TO A LOT OF REVISION AS I FIND OUT WHAT I LIKE/DON'T LIKE AND WHAT I PREFER IN MY INTERPRETATION (if indeed I prefer anything at all)
The shortest version though can be summed up as "complicated as fuck"

Miscellaneous Headcanon - Stuff that doesn't fit into another category yet idk I'm just writing things out as I think about them. There are some things I won't really touch on because they're too hot button of topics (things like ethnicity and etc.). The one exception to this is gender

Age - Around 10 or so, give or take a year
Gender - given that they're androgynous as all get out and are perfectly content with being referred to by gender-neutral pronouns, I figure Frisk views their personal gender identity and expression as "meh"