What evidence is there that Chara and Frisk are different people?

The way that I interpret the pacifist ending is that after Frisk/Chara restores everyone's memories by "reaching for their souls", he/she then finds his/her own soul (which Asgore would have taken when he/she died) and recovers his/her own memories. Chara now fully realizes that he/she is the same person as the first human. After the Asrial fight, Chara sees that Asrial is coming to terms with grief. Because of what had happened before, where Asrial put everyone's lives in danger, Chara does not want to reveal who he/she is, as it would freak everyone out and make Asrial return to his obsessive self. Chara decides to use the fake name "Frisk", to protect everyone.

I keep seeing people say that Frisk and Chara are different people, but I don't see how that would work (especially considering that Chara will keep whatever name you enter, when you do a genocide run). I see various theories about the game which state this like a fact. People who do this without giving any evidence can be a bit annoying.

What evidence is there to justify the theory that Chara and Frisk are different people?

Solution 1:

You are approaching this like there is only one ending, but really there are 5 main endings to get the full scale of what’s happening:

1. True Pacifist (the one you were talking about)
2. Pacifist (staying Level 1 but not befriending everyone)
3. Neutral where you kill some and spare some (this is the only ending with sub endings)
4. No Mercy (killing every encounter)
5. Genocide (Grinding to lvl 20)

The True Pacifist and Genocide endings reveal the most story. In the Genocide ending Chara Stands right there, and talks to you, Frisk. In the pacifist ending, it states multiple times, that Flowey has no soul, and Flowey is Asriel so it says “you feel something resonating within their soul, stronger and stronger” that is theorized to be Chara because again, Flowey does not have a soul, and Asriel is Flowey

Solution 2:

From the entry at Undertale wiki:

Chara, also known as the first human or the fallen human, is the first human to fall into the Underground. Chara is also the fallen human that the player names at the start of the game, and not the controllable character who is played throughout the entirety of Undertale.

Frisk is the playable character and main protagonist of Undertale. After Frisk falls into the Underground, they embark on a journey to return to the surface. Frisk is the last of the eight humans to fall into the Underground after travelling to Mt. Ebott.

So no, physically they are not the same characters. But yes, indeed Chara seems to "stir in your soul" when you are following the Genocide route. This does not imply that Frisk IS Chara, only that Chara is/can be inside Frisk as something within his SOUL. But in the Undertale having someone else's soul doesn't make you that person: Flowey has at one stage 6 humans souls yet he doesn't change his behaviour.

In the Genocide Route, Chara says that the protagonist's "determination" and "human soul" awakened them from death. There is evidence that some narration, as well as descriptions of certain actions and events, is by Chara themself. However, the narrator throughout the entirety of Undertale is up to speculation.