Stardust Woodsreplaces Snowdin Forest.Starlight Isles(Summer) replaces Snowdin (Winter).The City of Old is another brand new area, containing various shops and miscellaneous activities.The Sewer is a brand new area which contains an 'entirely new event sequence' in which Chara must dodge leaks of boiling water and later survive an intense overworld attack from the mad dummy.Ruined Home (Spring) replaces The Ruins (Autumn).In order to create 'new' areas, the seasons corresponding to each major location of Undertale have been swapped, and brand new sub-locations have been created. His catchphrase is 'Put some pep in your step!'.

Spep is the mascot of Team Switched and appears alongside the Annoying Dogs for shenanigans.

His personality is confirmed to take inspiration from those of The Snatcher from A Hat in Time and Antasma from Mario & Luigi: Dream Team.

Koffin-K is the count of Koffin Keep, and Crossbones's arch-nemesis.

Spooky Seltzer and Woodwork are two original randomly-encountered enemies from an older iteration of TS!Underswap, planned to be used for its current iteration.

The Greasers (Aaxel, Norman, Moldrick) are a trio of bullies based off of existing monsters from Undertale.

Mordax is an original randomly encountered enemy found in Stardust Woods.

Beenard is an original randomly encountered enemy found in Stardust Woods.

Syrose and Butchy are two original randomly-encountered enemies found in Ruined Home.

While the fine details of their roles are undecided, it is known that the 'BG' stands for either/both 'Body Guard' or 'Bouncer Guy.'.

BG01 and BG02 take the roles of Bratty and Catty.

Bratty and Catty take the roles of RG01 and RG02.

He owns the retro-themed fast food diner chain, Grillby's. Not much is known regarding this mysterious being.

Gastertakes the role of the River Person.

She's the leader of the Spider Scouts, who hold bake sales in Stardust Woods to raise funds for spiders. He is in love with Flara, causing The Greasers to hate him. He has very few similarities to the original Nice Cream Guy. He acts as the sole delivery boy for every Grillby's location in the Underground.

Bugerpant (Burgerpants) takes the role of Nice Cream Guy.

He's the Underground's top musician and DJ, with his own radio show sponsored by Undying Robotics.

Sir Scratchalot (Napstablook) takes the role of Mettaton.

She is the Royal Scientist and owner of the royally-sponsored company Undying Robotics. As Admiral of the Royal Navy, she delivers powerful speeches to the residents of Crystal Springs. He rescues Chara from the Temmie horde at the end of Ruined Home. There's only one of him, unlike the Temmies of Undertale. He is an aspiring artist and creator of the official Crossbones Comics. On the genocide route, he will prove himself to actually be very serious, and threaten Chara with a bad time. When off-duty, he is very laid-back and not very serious. Under the name Crossbones, he acts as a vigilance 'superhero' who seeks to capture Chara throughout Starlight Isles. He is an energetic, talented skeleton monster who supervises his brother's antics as a 'superhero' from behind the scenes.

He can be found performing for his adoring fans in The City of Old. He is an attention-loving ghost monster, known throughout Ruined Home as a star.

Mettacrit (Mettaton) takes the role of Napstablook.

They are a vindictive ghost monster possessing a training dummy, and act as the unwilling tutorial enemy for Chara, and a consistent antagonist throughout the Ruined Home arc. He is an old monster who acts as the caretaker of Ruined Home, providing guidance and assistance to Chara throughout the area. It is later revealed that they are only one of many, who form a hive mind of sorts. They are a seemingly abnormal cat/dog monster who encounters Chara at the entrance to the Underground. For unknown reasons, their words will sometimes change. Ebott for unknown reasons, this human child fell into the Underground.

Minor details have been added to the game based on unused content from Toby Fox's UNDERTALE (including scrapped concepts, designs, artwork, sprites, and music.).Battle segments consist of a bullet hell format, where the player must control a SOUL to dodge monsters' attack patterns. In the overworld segments, there are various puzzles, interactable encounters, and newly-added overworld dodging mechanics inspired by those of DELTARUNE.

TS!Underswap plays identical to UNDERTALE, where the player controls a human as they navigate through the Underground.

Underswap the game