First of all, fighters

Stray Child - a human that fell in the Underground. Their skin can be the Protagonist, the First Human, and the Protagonist dressed in 6 humans' armor, while their sweater may or may not loosely resemble fangame characters. 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. Among their outfits are references to their looks in Light World (for Ralsei, it's just Asriel's church clothing color) and Second Sanctuary, The Legend of Tenna's heroes' palette, the palette representing possible variations of Prohecy Heroes as different characters, as well as some other references I haven't decided yet.

Mii Fighters

Papyrus costume for the Mii Swordfighter. 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.

Lancer costume for the Mii Brawler.

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

Cyber City. BGM: Smart Race/Cyber Battle

Dark Sanctuary. BGM: From Now On/SPAWN

Assist Trophies

Lancer - Would ride the bike on the platform he was summoned, causing players damage at collision and occassionally throwiong some items.

Hammer of Justice - Would stand on summoning point and charge Double Buster, which he will shoot multiple times at ~6 second rate.

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 Swordfighter in Papyrus costume.

Undyne's Spirit Battle is represented by Lucina in red 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 orange outfit.

Chara's Spirit Battle is represented by Stray Child with Chara skin.

Lancer's Spirit Battle is represented by Mii Brawler in Lancer costume.

Noelle's Spirit Battle is represented by Zelda in white outfit, with Freezies spawning occassionally.

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

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

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

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

Queen's Spirit Battle is represented by Zero Suit Samus in blue outfit.

Tenna's Spirit Battle is represented by R.O.B. in red outfit.

Lanino and Elnina's Spirit Battle is represented by Ice Climbers in orange-and-violet outfit.

Old Man's Spirit Battle is represented by King Dedede in green outfit who summons Hammer of Justice Assist Trophy.

Boss Fights

The Roaring Knight!