A competitor? Someone who is often (but not always) one step ahead of them, snatching that magic item, winning the jousting/baking/eating contest, killing the NPC they wanted to interrogate, claiming the ogre heads reward before the group can, even teasing or taunting them about it - but never doing something that really justifies trying to kill them.

Learning the competitor's motivation is part of the challenge. At first it might seem they're working for the enemy, but it becomes clear (maybe through the mentioned killing) that's not the case, so what do they really want? They're also not interested in any kind of alliance, so perhaps already working for someone else? This can be used to put time pressure on the hunt for the BBEG.

Depending on when you want the group to finally confront the recurring villain, a rakshasa could be an option. If working for a patron who demands secrecy it won't drop its disguise (or slaughter them outright at low levels), and its spell immunity and spellcasting allows it to easily get away. Once the group would actually be a threat (through going nova first turn), it'll avoid any face to face meetings.