Question

do they finish off your opponent's pokemon when they're at low HP or something like that? and would it most likely be a good idea to have one on my wifi team?

Answer 1

They do what you said, but also come in when one of your pokemon faints, to"revenge" kill the opponents pokemon. It is always a good idea to have one on your team. In summary, revenge killers pick up the scraps and finish off weak opponents. Hope this helps!