Fuck it I wanna write some undertickles headcanons for the sake of getting them out of my head.
Tickle headcanons for my faves (Alphys, Undyne, Mettaton, And Sans) below the cut:
Alphys:
I think she's 100% got a secret tickle blog, okay. I think she is an easily flustered lee. 100% lee, maybe 95% at LEAST.
Would rather die than admit she has a tickle blog. It's a very popular blog, though. Mostly posts fics and anime scenarios, sometimes posts irl-type tickle stuff she wishes would happen to her.
She can't even say the word, she stutters around it or avoids it entirely with synonyms ("feels weird" ect). Mettaton lightly teases her about it.
The kind of person who can't watch tickle scenes without blushing and looking away. Rewatches them over and over in private, but screamed so loud and died inside when Undyne walked in on her once.
She's got really cute high-pitched giggles that sometimes slip into a snort that embarrasses her. She'll let out a surprised little squeak about it and try to cover her face. VERY succeptible to teasing of any kind.
Ticklish just about everywhere, but her stomach and ribs are just a tad worse. She'll sometimes try to curl up when these spots get targeted.
Gets targeted the most by Mettaton (before he was too busy as a star) and Undyne. Sometimes targeted by the amalgamates when they're feeling playful.
Cannot tickle someone else to save her life. She gets too flustered after only a few seconds. Her claws would probably be deadly if she could just lock in.
Undyne:
100% a ler. She's in it to win it. She loves grappling and playfighting, and she's strong enough to pin someone in only a few seconds.
She'll use her claws to scribble around at any vulnerable spots wildly, but will also squeeze with her strong hands. Can be a little rough at times, but never enough to hurt anyone.
She's incredibly good-natured about it. The caveat is that she will comment on any noises the lee makes and will lightly mock them for being unable to defend themselves against such a silly assault.
She's not afraid to bring up their devastating loss in front of others, ESPECIALLY if they give her an attitude. "Huh?? What that, punk!? Maybe I need to scratch your ribs and remind you who the tougher one between us is!" Keep going and she'll demonstrate.
Loves to Target Alphys, because she is cute. Loves to target Papyrus, because "it's training, you have to be ready for anything!"
She's not very ticklish, but that doesn't mean she's invincible, either. The webbing on her ears and her neck can get a chortle out of her just before she snatches you for revenge.
Will let Alphys tickle her, because she only lasts for a few seconds before getting flustered.
Did find Alphys' tickle blog once. She thought it was cute. But she thought Alphys getting super flustered about it was even cuter (and kind of hilarious).
Overall she just thinks tickling is fun and kind of cute. She likes that there's a cheat code for making people laugh.
One of her favorite ways to cheat at playfighting is to use her magic to summon those arrows. They'll poke at a lee from every direction and weaken their defenses enough for her to finish pinning them and REALLY go to town.
Mettaton:
He likes to pretend he's strictly a ler. He got cocky because of his base robot form's high defenses rendering him almost invincible, which meant he could launch playful attacks with no revenge. That changed when he got his new body, but we'll get to that later.
He loves tickle hugs. He can stretch his arms reeeeeeeeal long, which will wrap around his lee to prod at sensitive spots in reach. He'll coo at their reactions all the while, and he's plenty strong enough to keep them from collapsing, which means that you can't escape him. He just thinks all those squeaks and squeals are soooo precious!
He'll sometimes tickle someone on camera with a quick prod to the side, a finger slid up the back, just for a quick smile for the camera. He prefers the bigger attacks off-camera, because he wants to keep all of those adorable reactions all to himself.
His magic can make little tickly shocks, but his hands are also capable of vibrating. It's exactly as effective as you'd think.
Overall stronger than he looks, and more than capable of keeping someone trapped for tickles.
BUT. When it comes to his EX body, he is VERY much vulnerable to tickles. He even surprised himself with how sensitive he suddenly was. He doesn't HATE it, which surprised him.
His most sensitive spots are all clustered around his soul. Stomach, hips, and thighs. These spots make him squeal and babble nonsensical pleas of mercy. He's ticklish all over, but these spots in particular get him badly.
You can make him flustered enough to short out if you return his exact style of teasing: cooing over how cute his reactions are. point out how his tickle laugh is more cackly than his TV practiced laugh and he'll weakly hit your shoulder.
Sans:
He's a little too lazy to be a very effective ler. He prefers to be opportunistic rather than like. Hunting his lee. He'd rather tag someone when they were sleeping or otherwise distracted.
As such, his tickles are actually pretty gentle, all things considered. Sometimes he gets a good squeeze in, but poking and prodding are his preferred methods.
He thinks wake-up tickles are funny because he likes to watch their expression change from being peacefully asleep to confused to helplessly giggly. He thinks the helpless confusion is kind of cute and hilarious, especially when they're waving their arms around uselessly or giving sleepy little pleas of mercy.
Puns. He makes SO many puns while tickling. "Hey, I'm just ribbin' ya." While prodding at their ribs. "Was the joke that... side-splitting?" While skittering on their sides ect ect.
Usually just targets Papyrus. Later targets Toriel (or tries to, at least).
Is surprisingly difficult to get revenge on. Because of the warping thing. If you take too long to recover and lose sight of him, he's GONE. If his lee wants any chance of immediate revenge, they have to snatch him up immediately.
He tries to play it cool when he's finally cornered or caught for revenge. Don't be fooled. He's panicking SO hard on the inside. He's easy to catch if you sprint at him before he gets off-screen. He doesn't like to run.
Anyway. If you wanna destroy him immediately, go for his ribs. He'll slip into an almost silent wheezy laugh. His spine is also a spot that can get him wheezy.
He'll squirm around, but he won't otherwise fight back. For how ticklish he is, he's surprisingly good natured about it. He'll try to wheeze out a pun or two (as long as you aren't on his ribs).
Sensitive to softer textures, like feathers or fur.
Won't go for revenge. He's too busy falling asleep afterwards.
