I was wondering, are there any characters in Arknights that on the surface are jokesters, but anre actually immensely deep and powerful? I know about characters like Jaye and Nothing, but they aren’t particularly funny and casual. What I’m looking for is someone like Sans, basically.

"Nothing isn't particularly funny" oh we're just fundamentally different people then because I smile like a babe listening to jingling keys every time he's in the scene.

In the exact same regard as Sans, 1:1? Not really. Similar enough in some regards or in essence that they warrant mention? Yeah, I'd say so.

Aak is a good case, I'd say. Aak is a very casual guy, perhaps excessively so, referring to Doctor as "my dude" and in general having a very whimsical whistle to his steps, not to mention his seemingly jovial soul. Then you dig a bit deeper and you realize, well, despite the way he behaves, he takes what he does extremely seriously and is one of the most ruthless characters in the cast as a whole, in addition to being a medical sciences prodigy the likes of which Terra has seldom seen, and being infamous in the criminal underworld as a vigilante and executioner that has no qualms disposing of a mother fucker, if said fornicator of moms is a criminal and adversely affects others. See, the thing with Aak is that he, for the longest time, hated being so good at the medical sciences, because as he saw with his own twisted father, a brutal underworld doctor and criminal, curing illnesses doesn't begin to truly rid the common man of their suffering: There are always oppressors, abusers, those who selfishly make lives difficult for others for their own gain. Aak believes that to be a truly good "doctor", he needs to eliminate the root cause of the people's suffering: Criminals that will hurt them. Since his cooperation with Rhodes Island, his views have shifted a bit for the better (understanding people better, finding a good friend that shares his latent curiosity and love for the medical sciences and research in Warfarin), but it's still evident that Aak remains an unstable vat of fluoroantimonic acid waiting to bubble over, if the right trigger is present. He's not particularly powerful, with all his evaluations being "Normal" and "Standard", but his ruthlessness, his knowledge of the underworld, and his deep knowledge of medical sciences do make him quite dangerous despite "my dude"ing you.

Ceobe is the other one that comes to mind. Ceobe basically replicates the experience of having a big dumb loving dog, not just with Doctor, but with others as well, such as her canonical friends Vulcan and Sesa (Sesa being someone that also qualifies imo), and is in general a fun goober that livens up every scene she's in on virtue of, well, being a big dumb loving dog with all that entails: She WILL break into the kitchen even though she knows it's forbidden for her to do this, she WILL steal food, she WILL whimper and apologize, she WILL grow immensely defensive over her loved ones over things like "a loud vacuum cleaner" and WILL act over any perceived threat with maximum power; the entirety of Integrated Strategies 1, Ceobe's Fungimist, comes about because Ceobe gets high on hallucinogenic mushrooms she found in the jungle, and goes apeshit because she imagines this whole scenario where Villains have kidnapped Doctor and only she and whatever friends she can recruit on the way can save them, resulting in her beating the absolute shit out of numerous warrior tribes Dynasty Warriors style. Let's talk about that last part! She beat the absolute shit out of numerous warrior tribes Dynasty Warriors style while high as fuck on shrooms. She can do that! Because Ceobe is actually fucking shredded. Despite being a Funny Dog, Ceobe is a legitimately Arts genius, having no formal training and yet being able to use Arts with no problem, almost instinctively, as well as simply being able to harness pretty much any weapon she touches and empower them further with her Arts. Part of this is definitely because she's VERY Infected, but not even that explains just the sheer magnitude and expertise with which Ceobe seems to use her Arts. And speaking of? She dragged herself, across much of Terra, while incredibly Infected and with no care at all.

Ceobe, by all rights, should be dead. Not even the Medic Operator that wrote her medical file has a lot of faith, and her Oripathy is explicitly very grave and only getting worse. And even in this state, she dragged herself and the small arsenal strapped to her back across the world.

Her Physiological Endurance rank of Outstanding is no joke. Keep in mind, this is a rank equivalent to the toughest and most resilient characters we know of, such as Specter and Hoshiguma.

This is all stuff you'll only ever realize about Ceobe if you pay attention, because she's almost exclusively used for comedic relief, but the funny dog is actually a natural archmage and weaponmaster that just won't fucking die, if we were to use more fantasy adjacent terms for her.

There's more (like Sesa) but those two are the ones that jumped to mind.