well, no. but I feel like it would be a good side character in chapter 3, because WHY NOT!? you encounter frisk like spampton, but you can also find him in a room, NOT a shop, like jevil.

you find him/her in the movie studio, somehow playing undertale, with ralsei and susie. he/she is in hotland, battling a Tsunderplane.

her/his dialogue when encountering is:

WHOA!!

Susie: uh... sorry to bother ya, but, are you a lightener?

lightener? what's that?

(also how did you find me?)

Susie: Y'know, people and monsters from the light world?

Doesn't ring a bell...

Susie: ...

Susie: well, LOOK. you're a human.

Susie: You started out in the light world.

Susie: You're in a dark world now.

...

ok...

*sigh*

I think I should tell you my story.

the screen cuts off black, having a moment where it's what you see when you open up undertale, but more lines after what it normally is are:

it was my story. it was my tale.

it was my UNDER TALE

it was hard to survive, but no matter what i did...

where i went...

i never lay a finger on ANYONE.

i never gained LOVE, but i gained love.

after my adventure, humans and monsters lived in peace.

but...

it wasn't over... YET

the race next to even humans and monsters...

YOU taught me that.

darkeners... lighteners...

there was a whole world waiting to be discovered.

(screen comes back to the overworld)

today, i've found a simulation of my life, called Undertale.

It was made by my race... with a little help from Alphys, since she saw my whole adventure.

It's a very, VERY popular game now, and I heard it has three endings, and even fangames!

Susie: so... it's a simulation of your whole LIFE?

Yep.

Susie: cool. is it on Xbox?

double yep.

Susie: alright. hand me a digital copy when you're ready.

That's the problem...

Susie: huh?

I CAN'T GET OUT OF HERE!!!

ralsei: the dark world?

YES!!!

I'm trapped here, I don't know how to get out, and-

Ralsei: We just have to seal the dark fountain.

...

oh.

can I come with you?

Susie: uh... sure! anytime y-

ALRIGHT!!!

*Frisk joined the party!*

oh, I almost forgot the game!

*Undergame became one of your key items.*

AND that's my ultimate Chapter 3 theory... PART 1! if you want a part 2, then leave a comment saying "part 2" or "more" in this post!