Could an OC manipulate mercury like Gaara manipulates sand or would that be OP?
So, the first thing to mention is that Gaara uses an advanced nature transformation, the [Magnet Release], to manipulate sand. It would be fine to have an OC that has this kekkei genkai, especially if they belonged to Suna or Kumo.
The thing to understand about Magnet Release is that it converts the user’s chakra into magnetic forces. Using this, the user can apply the forces to various object(s) that they want to control or transfer the magnetic force to other people through an object they were controlling. Something to mention is that the majority of Magnet Release users typically apply their kekkei genkai to things that are magnetic—the Third Kazekage used Iron Sand; Gaara uses a “sparkling metallic sand,” according to a novel; and some guy from Kumo magnetizes special shuriken.
Then, we have Rasa.
Rasa uses Gold Dust with his Magnet Release. Pure gold in bulk is not magnetic. Well, not in the [same way], say, iron is. Instead, gold (in bulk) exhibits something called [diamagnetism], which basically means that, when a magnetic force is applied to it, it is repelled instead of attracted. However, nano-sized pieces of gold are magnetic in the same manner iron is—that is, they are ferromagnetic. But, despite being made up of small gold flecks which are ferromagnetic individually, he ends up using his powers to create gigantic constructs of gold—which would then technically be gold in bulk, meaning it would lose its ferromagnetism.
Meaning Rasa shouldn’t be able to magnetize gold like he does.
Yet he does.
I bring this up because it’s been speculated that Rasa has a strain of the Magnet Release which allows him to magnetize diamagnetic material. This is apparently only seen in him.
And this brings me to answering your question. Finally.
Mercury is not magnetic. Only at room temperature is mercury very weakly diamagnetic. And only Rasa has been shown to be able to utilize diamagnetic material through his kekkei genkai.
I don’t recommend making your OC be able to do the same, unless they were somehow related to Rasa. It would probably be best that you either find a different material to use, or use mercury in another way.
-ATB