The process of creating Gems uses Quintessence, which is itself later established to be the blood of Celestials.
The Trolls of Trollhunters originally came from Svartalfheim, home realm of the Dark Elves, who enslaved them. This led to the trolls aiding the Asgardians when they fought the Dark Elves during the Convergence, for which Malekith supposedly wiped them out.
Kassandra is one of Ares' demigod children, who ascended to full godhood after completing her quest.
Bellroc was the first Sorcerer Supreme, with Skrael and Nari also holding the title at various points.
Juleka is from a long line of witches on her mother's side.
The First Ones who colonized Etheria were Alteans who used alchemy to permanently lock themselves into a form lacking their shapeshifting, ears, and facial markings in order to hide themselves from the Galra, said modifications even passing onto their children.
SFIT was heavily invested in by Stark Industries. Also, Hiro and Tadashi's parents worked for the LIFE Foundation, and Granville and Callahagn taught Tony when he was in school.
The humans on Etheria and other worlds in the Milky Way are the descendants of escapees from Blue Diamond's Zoo and the Xandarians who rescued them.
Monica has her powers by the time of The Royal We despite the Hex not existing yet, if at all. Word of God is that this is the result of being exposed to Carol's Tesseract-derived powers over her childhood, which altered her DNA.
Señor Uhl retired from active S.H.I.E.L.D. service after a mission in Bahrain, implied to be the same one from Melinda May's backstory.
The Glitch Techs speculate that the Glitches are the result of Arcade's powers and technology, though they can't determine whether it was a deliberate act or not.
Nora Wakeman used to work with S.W.O.R.D. Additionally, in contrast to how Jenny is normally Nora's creation, she's instead revealed to be a Cluster Superweapon that Nora stole from them.
The mage who created the Miraculous was Agamotto himself.
She-Ra was originally Etheria's Sailor Guardian, tied to the planet's Quintessence, before being modified.
