UltSans is GODLY good as vessel, thick, and horny. But is also... Allergic to dust.
Yes, is allergic to dust.
From all the vessels known so far, UltSans is allergic to dust.
UltSans is a gamer, which would do a good friendship with Berdly.
UltSans, the Player, and Berdly, would be the Gamer Trio.
Until berdly starts being analitical and share stadistics information.
When that happens, UltSans slowly turns to look at the red heart soul, and then nods. And then gets up, grabs Berdly, and throws him away, before closing the door locking Berdly away.
UltSans: "Well, where were we? Oh yeah! About the bug... Somehow it works. You roll here, jump above the barrel, slash it, and it somehow takes you there. Don't aske me how, or why, 'cuz I don't know, but works."
The Player would be actually close to UltSans, being the possible best vessel the soul would take by predetermined vessel options.
And both would actually have a good friendship.
The Player, controlling and helping UltSans for the player's benefit and entertainment, which already gets 2 points of interest from UltSans to be met.
UltSans, who already ready as vessel, and after all the lessons learnt, likes and wants to be controlled.
If this scenario happens, UltSans would be happy, as could be using his magic even when he thought he sacrificed it in exchange for determination when changed his soul, happy to be entertaining someone, relieved to not worry of options or what to do as is very used to be a puppet and a vessel and doesn't know how to take decisions.
For fun annoyment, if UltSans went with Kris and met Berdly, for just annoyment and entertain while making fun of Berdly; UltSans would ask if that blue bird was Kris' pet.
UltSans will always tell and think he is more of cats than dogs, but in reality, enjoys way more dogs than cats, as mimics kitten noises, dog barks, and playful dog's behaviour when are little dogs and not big dogs as is just to mimic and play with them in a more controlled way.
UltSans would see Toriel as smashable in some universes and timelines, as doesn't have memory to make him know is technically Asriel's reincarnation or reborn. So in technical terms, UltSans does like to have adult intentions with his technically speaking, mom and sibling; which means he is northern.
If Frisk is a female adult[1], UltSans sees her disgusting and just a bitch.
But if Frisk is a kid, UltSans doesn't think and is more friendly or merciful.
So the younger Frisk is, the more merciful UltSans will be.
I just hope no one gets the wrong term for this, he's not a pedophile. 😅
But if UltSans meets Undertale Toriel, Altertale Toriel, Alterfell Toriel, Alterswap Toriel, StoryShift Chara, StoryFell Chara; UltSans will flirt before thinking for other options.
The only Frisk UltSans doesn't hate, is Core Frisk. But UltSans actually takes care of Core, as if he was their big brother.
GlitchFlame, the name for the ship of "UltSans x StoryFell Chara". This ship is actually canon, but not all versions have a link to this ship.
Actually, by default, base Fanon versions[2], are attracted to Fell AUs, mainly StoryFell for StoryFell Chara. But this attraction is not only for visual, that it is mainly, but because the behaviour and control emanate from them, a control UltSans is familiar and feels warm as a home.
Canonically speaking, StoryFell is UltSans' favorite AU.
But when FellShift Gold enters in the game, that AU is his new favorite, finding FellShift Gold Chara and Muffet having quite lot of 'trama', and finding the ending quite miserable and sad to happen but still expected coming from humans, and Toriel as more than a warrior and respects her more than could respect other characters.
[1] Optional: with huge physical attributes
[2] This means all who are "normal" and are Fanon versions. Does not count special Fanon versions as D2V, Ashed, NM!UltSans, or any other version who doesn't think and FIGHT is main and only option.