Most of all these are about Mario because he is the main person that I watch, so let's go.

OoO Headcanon- Mario is shorter then Kay and that Kay like a lot of snake can make they self taller be having less of their tails on the ground.

SCP theories- Mario is the son of Thanatos.

So in the episode were they meet OoOKay, after Mario comes back from being dead (I think) and asks him who he is the son of and Mario said Jonathan and I do not reminder what his mother's name is.

And do you know the name of the guy that Mario meets when he die, IT IS JONATHAN.

SCP headcanon- Mario has a little sister and a brother

OoO Headcanon/theory- Mario's mom is the descended from Posideon. Mario can talk with horses, and the only other person we know that can do that is Percy.

FTo Headcanon- I think of Earthas as a mix of McGonagall and Mary Poppins, also that Earthas was the guarden for the forest she raised Mario.

SCP headcanon- Mario can not age and also I think it would be cool if like on Mario arm he had the number of time he as died.

That is all I got of now

~Ace out