Onion is a quiet character. He is also a "supposed human." In my opinion he is not human. In the series it is repeatedly restated that " he is not human" by Steven and Ronaldo. My theory is that he is actually a gem. A gem that did not participate in the war maybe he decided not to take sides. In " the onion trade" amethysts gives Steven the replicator wand and says, "you're magic I'm sure you can figure out how to use it." Onion is a "supposed human" yet he figures out how to use it and even levitates a car. Another thing is that humans have ears. Gems can choose wether they want ears or not. I think that onion is pyrite. Pyrite is "fool's gold" you think that you have found something valuable like yellow diamond(which most of the fandom thinks) but you just found something average. Onions peel in layers, pyrite flakes like layers. Pyrite is found in nuggets and onion is tiny. The metaphysical properties of pyrite is power to control, power to manipulate a parent or employer, lending the power to resist without becoming angry or upset, changing the balance of power. We have seen onion getting screamed at for doing something rebellious yet he keeps a blank face and shows no emotion. Pyrite also is excellent to wear or carry as an amulet to deflect harm and danger.It is especially helpful when one is away from home or performing hazardous work. Onion does reckless stunts and is never harmed. Of course he would be a gem away from home. Dedicated to @Kamikaze!