First of all, fighters

Stray Child - a human that fell in the Underground. Their skin can be the protagonist, the first human and six other humans with original design consulted between Sakurai and Toby. Regardless of the skin, the child can use almost all weapons from UNDERTALE, as well as doing "friendly" taunts. Their Final Smash represents a huge knife slash, referencing Genocide Route ending.

The Fun Gang - group of heroes from DELTARUNE. Just like with Pokemon Trainer, the player can toggle between the warriors anytime. Most of their special moves are re-used from original game. Their Final Smash is summoning of Thrash Machine.

Mii Fighters

Papyrus costume for the Mii Swordsman. He wields a long bone instead of sword, and his hat represents his iconic facial expression with open mouth. Sadly, it won't hide Mii's true face. The costume comes with a Bonetrousle remix.

Stages

Last Corridor. BGM: MEGALOVANIA (Ultimate Mix)/ASGORE

Snowdin Town. BGM: Bonetrousle (Ultimate Mix)/Dating Fight!

Field of Hopes and Dreams. BGM: Field of Hopes and Dreams/Vs. Lancer

Spirits

Flowey's Spirit Battle is represented by Piranha Plant in yellow outfit.

Toriel's Spirit Battle is represented by Zelda in black outfit.

Sans's Spirit Battle is represented by Mii Gunner in Sans costume.

Papyrus's Spirit Battle is represented by Mii Swordsman in Papyrus costume.

Undyne's Spirit Battle is represented by Mythra in Pyra outfit.

Alphys's Spirit Battle is represented by Isabelle in blue outfit.

Mettaton EX's Spirit Battle is represented by Samus in purple outfit.

Asgore's Spirit Battle is represented by Bowser in blue outfit.

Lancer's Spirit Battle is represented by Ludwig Koopa.

Noelle's Spirit Battle is represented by Zelda in white outfit.

Berdly's Spirit Battle is represented by Falco in white outfit.

K. Round's Spirit Battle is represented by King K. Rool in orange outfit.

Spamton NEO's Spirit Battle is represented by Dark Samus in yellow outfit.

Sadly, I can't come up with Spirit Effects.