basically, there’s a theory that chara provides all the narration text. this is supported by many pieces of evidence. the narration always refers to frisk in the second person ( ‘it’s you!’, ‘froggit attacks you!’, ‘you win!’, etc etc ), except during a no mercy run. by then, it’s canonically chara that is in control. the narration switches to first person ( ‘it’s me!’, ‘in my way’, etc etc ). in no mercy, the narration also describes various items in asgore’s house in first person too.
the narration also provides the player/frisk with lots of information that frisk would not know, most notably through the ‘check’ option. chara would know that to peacefully beat a loox, the solution is to ‘don’t pick on him.’ or that shyren is ‘a talented singer, with a little help.’
the narration also translates for the froggits. frisk would not know how to beat these monsters or the froggits’ language. chara, in their long time spent underground, would.
there’s other pieces of evidence, but those are the most prominent.
I am Silver Tongue, I am an artist. I have many characters and you can check out my art in the art tag. I occasionally practice witchcraft though I don't do anything too complicated. I am girl 2 and don't know what else to put here.