YMMV / TS!Underswap

• Awesome Music: Check here.
• Iron Woobie: Asgore. Incredibly kind and cheery, he makes the best of his situation by offering his help to other monsters and is so loved for it that Bugerpant even asks Chara to be good to him because he can tell Asgore's been through a lot and deserves something good. We haven't found out all the details of his past yet, but from what we do know and suspect, he's right on the money. Not only is Asgore now separated from his son Asriel as well as his wife, but it's suggested by the creators that he has never even really gotten to meet Asriel, implying that the separation happened when Asriel was too young to know him (and possibly not even yet born). With the swap, he likely adopted Frisk and Monster Kid before that, and then lost them both in the same day, with it implied that Chara's arrival happened on the anniversary of this terrible day, making him want to save this child to make up for all the other human children who've left his care and died. Chara either turns out to be a murderer who butchers many of his citizens and friends and who he dies trying to put down, or they leave despite Asgore wanting to be like a family with them. Poor Mr. Dad Guy...
• Player Punch: Doing the Ruthless route as per Undertale tradition, which has you killing all the cast of really lovable and sympathetic characters for no reason except your own murderous intent. But there are instances that top most of the others.
  • Killing Asgore. Even as he lays dying, he pretty much tells the player that killing him served no purpose but to slow them down, before giving you an overly polite 'go to hell'.
  • Killing Harry and Larry in the Ruthless route. The two are outright terrified of Chara after their failed stunt and will run away after meeting them again. The whole sequence then involves chasing them as they reach a cliff, without way of escape for them and as they bicker, the player is given multiple options to either back down or walk forward, scaring the two even further every time the latter option is chosen. And then it goes into the two going in a last ditch effort to save themselves by fighting Chara, though they still look terrified during the whole fight. To win you have to weaken Larry and then push him off a cliff with Harry trying to save him but being too slow. The guy then kneels down giving up, leaving the player to finish him off. It's not unheard that players felt like they were killing people and felt extremely bad. Worst is, the whole sequence is entirely optional since they're no longer an obstacle. You cornered and killed them because you wanted to.
• Unexpected Character:
  • Flowey popping up at the end of the first demo to try to defuse the situation with you and the Temmies, with the implication he's going to be as important in the full game as he was in Undertale.
  • Muffet appearing halfway through the Starlight Isles, rather than the Hotland-equivalent of Boreal Bluffs. Her fight isn't any easier just because she appears earlier in the adventure either, bringing a new bag of tricks on top of the introduction to the Purple SOUL mechanic.
• The Woobie: Chara. They are hinted to be a victim of Demonic Possession in all routes, either forcing them to abandon the place where they want to live with Asgore and the citizens of Ruined Home and face more danger, or turning them into a cold killer murdering the people they could have befriended. And because the legend of Mt Ebott is still true of this world, like Undertale's Chara, it's implied in Starlight Isles they came to the Underground for an unhappy reason.