First, think about their motives. A great villain often has a strong reason for being bad, like a past injustice. For example, if your OC was betrayed by a Hunter in the past, it could fuel their hatred towards Hunters. Second, design their Nen ability. Make it unique and related to their character. Maybe a villain who can manipulate shadows as a result of growing up in the dark alleys. Third, give them a distinct appearance. It could be a scar across the face or a unique style of clothing that sets them apart.
The biggest villain in Ice Hunt was Huang Zongwei. Huang Zongwei was a very strong anti-criminal investigation ability and a ruthless and decisive character. On the surface, he was kind and friendly to people, but behind his back, he was engaged in illegal activities that harmed society. Huang Zongwei's character had two faces and a huge contrast. It required the actor to have a strong ability to enter the role and interpret it. Zhang Songwen had a high degree of overlap with Huang Zongwei in the growth experience and style of the character, and his performance left a very deep impression. Therefore, Zhang Songwen could be considered the most suitable candidate to play the biggest villain in Ice Hunt.
Avoid the 'pure - evil - for - no - reason' trope. It makes the character one - dimensional. Every villain should have some sort of motivation, even if it's a twisted one. Also, don't just copy existing villains' powers or personalities directly. It lacks creativity. Another trope to avoid is the 'instant - redemption' at the end. If your villain has done truly heinous things, it should take more than a simple change of heart to redeem them.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.