Marvel is Klaus Mikaelson and Hayley Marshall second daughter which means Hope's Twin baby sister. After their Aunt wanted to take the girls Klaus decided to separate them and hide them. He couldn't keep both girls in the same place.
He had to make a very big decision which daughter to send away and fake her death and which daughter to hide from the world.
Cause two most powerful kids will draw attention of all the supernatural creatures but also Hunters. So he decided to send away.
He choose to send his younger kid away and fake her death while he kept Hope and hid her.
Marvel was found by Bobby on his motel room door. As much as he didn't want a kid he couldn't leave her there on the doorstep like she never was with there. So he adopted her, making the girl his daughter.
After Bobby death Marvel stuck around with The Winchesters. What happened when Lucifer's son is born? And with that comes that The Mikaelsons find that Marvel is Alive?
TO×SPN