So far I've only gotten so far to flesh out little bits of information about Sans, Papyrus, Gaster and Metaton.
Keytale
Except for ghosts, all monsters are born with a special "lock" on the most visible area of their body that is often difficult to disguise, which is usually the forehead. The material in which these locks are made of depends on monsters species, (for skeletons it is one type of metal).
The keys for these locks appear somewhere in the world, there is only one key per monster ever, it cannot be replicated. All keys look identical, it's only the magic imprint on each key that connects said key to a specific monster lock. The only unique key in this universe is Metatons keycard due to Metatons artificial body (Alphys did not make the keycard, it appeared just like all normal keys).
Damage to a lock, depending on severity, has to be treated the same as any human head injury, it is extremely difficult if not impossible to fix a heavily damaged lock.
A complete destruction of a key erases the connected monster from existance.
Also, it's important to remember that the only true ways for any Keytale monster to die is by unlocking their lock, destroying the correct key or by completely crushing their whole body into a unfixable state; in all other mortality-endangering cases the worst that can happen is permanent coma. This means that all Keytale monsters are essentially conditionally immortal.
Sans - Key
Keytale Sans is a grumpy and serious guy, he has some physical disabilities and a few missing fingers, and he can't walk properly due to missing a knee joint.
Despite being brothers he hates his Papyrus, possibly because he was always the scapegoat for Papyrus's childhood mistakes.
He is the older brother (in his late 30's), he despises Gaster, and he is approximately as strong as Underfell Sans.
He is not the judge of his universe, in pacifist and neutral routes you would encounter Undyne in the judgement hall and in genocide route you would encounter Asgore and due to undisclosed reasons once you defeat Asgore in genocide route you would fight Key in Asgores throne room.
He likes motor oil as a consumable.
Lowkey (pun not intended) inspired by the sun, the myth of Midas and the girl in the closet.
His lock is golden and magic is yellow.
Papyrus - Lock
Keytale Papyrus acts a little like a spoiled brat due to his general upbringing, he has major physical disability being permanently wheelchair bound.
He loves his brother Sans and wants to have a better relationship with him, but for general safety reasons he lives with Undyne who is his primary caretaker.
He doesn't remember Gaster but used to really love the older skeleton, he never learned what Gaster did to Sans whenever he made a mistake.
He neither can nor wants to be the royal guard, much rather prefering to make clothes for other monsters.
He likes sewing and doesn't eat.
Lowkey inspired by the moon.
His lock is silver and his magic is dark blue.
Gaster - Chain
Not much is known about Gaster, only Sans remembers him.
He used to be a mad scientist, he had a problem with favorism, spoiling Papyrus while abVs1ng Sans.
He was the sole cause for Sans's disablements.
His lock was copper and his magic was green.
Metaton
Largely the same old Metaton.
He is a ghost in a artificial body, he gained his keycard automatically once he posessed his new body.
Instead of Undyne, he is Monster Kids idol.
His lock is a keycard reader and his magic is purple.
