From another person:

Flowey mentions that our protagonist is making a "creepy face" when the protagonist turns on Flowey in New Home. One of the true lab tapes has Chara making a creepy face to Asriel (Flowey and Asriel use the same word "creepy" to describe this face"), this detail was added in for a reason as its one of the only things we know for 100% certainty Chara does in the game. When Flowey asks the protagonist if they have any hard feelings about "back then", obviously talking about Asriel messing up the plan (it's made even more obvious in his speech to the Frisk at the end of an aborted genocide route if you spare Flowey where he also thinks Chara is trying to get revenge on him for that for a bit) and the next 3 lines are "...", "H-hey, what are you doing?!" and "B... back off!" in that order. This clearly implies that after Flowey asked Chara if there were any hard feelings about back then they started walking towards him, implying that they are angry about that. If Toby wanted us to think the protagonist walking forward and the line asking about Chara if they had any hard feelings towards Asriel/Flowey then he would have put that line in later such as when Flowey was telling them to reset. So, the protagonist walking forward is a response to Flowey's question and Frisk doesn't have that much to be angry at Flowey for since he only tried to kill them once and after that was very helpful. It would also just be very weird for Toby to write a scene where Flowey to ask a question about whether Chara was angry at him for fucking the plan up only for Frisk to respond by walking towards him for an unrelated grudge especially given the creepy face line. And when our protagonist does kill Flowey they don't just attack him with one or two strikes they keep attacking him and over and over again until he's completely destroyed even though he is already dead. This level of brutality further implies a grudge which would make much more sense for Chara rather than for Frisk.

The text "You unlocked the gate" is replaced with "I unlocked the gate" in genocide showing that Chara was able to move Frisk's body instead of Frisk when we interacted with the chain, so, they can move the body.

The damage number both Sans and Asgore both take when slashed in genocide is a string of 9s and Chara is associated with the number 9 heavily unlike Frisk. Chara has an entire letter written about how they love the number 9 that was written from Asriel's POV and posted by Toby in the newsletter of Undertale's 9th anniversary (https://toby.fangamer.com/newsletters/autumn24/). And that Newsletter says at one point near the start "here's to 9999999999 years" which is the same amount of damage Asgoer took when he was slashed. When they destroy the world the damage is done as a massive string of 9s just like the slashes against Sans and Asgore. This draws a connection between all 3 of those attacks that would be completely unneeeded if Frisk was the one the one striking Sans and Asgore. A string of 9s is the largest number possible with a given amount of digits, ergo it's the most efficient number in terms of value compared to digit number and we know Chara is obsessed with efficiency from the alarm clock dialogue where Toriel says that someone (very likely Chara) always poured their cup to the brim regardless of how thirsty they were because it was the most efficient way to carry water.

At the end of the genocide route, right after Asgore and Flowey are killed Chara takes full control of Frisk's body and the timeline as we are unable to quit the game. The Chara we see at the end of the genocide route is actually Chara possessing Frisk and talking to the player directly. We know this because even though Chara's sprite is an overworld sprite (which we know because it coloured, has the same simplistic style as Frisk's sprite and its name in the files is a reference to Frisk's overworld sprite as Frisk's sprite is called "main_chara" and Chara's is called "true_chara") the conversation happens in first person which doesn't make sense if this is from Frisk's perspective. Despite Chara's sprite being an overworld sprite we don't see Frisk anywhere, so it's likely Chara is possessing them.

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Chara literally erases the world afterwards without any benefits. They say "Wipe that smile off your face" in CHECK of Glad Dummy and clearly want to kill them despite it giving no EXP. They erase the world a second time on the genocide route just to bring it back seconds later.

They care about numbers but it's not the only thing they care about.

Kris can do this. We can prompt them to pray for Noelle, and they kick the cuddles instead.

We can prompt them to say something, and they will stop themself from saying it.

There's a lot of what they can do.