First, is the life native to Pluto, or did they come from another cold world planet in another system? Do they have good tech lvl? Do you have any thoughts on it your self?
If they are not native, I would have them be an other civilizations failed try at FTL or Hyperspace and they just ended up getting stuck on Pluto many ages ago. They may have even known that earth had life but was never able to communicate with them because of the difference in tech ( their communications always got confused with static background noise to or sensors), or they didn't want to contact us because they see us as war like and didn't want to become slaves, they are war like and were trying real hard to communicate with the home world to get aid to take over Earth, they are a noble race and wanted to let us progress on our natural path with out their intervention, or what not.
I think I would have them live in caves/tunnel systems that they make. This might help them protect them selves from what solar radiation that would be around out there. On the other hand they may have adapted, and their bodies may be efficient at collecting solar/cosmic radiation to help their bodies process their food, and to keep them going. Kind of like a lizard sunning its self.
As far as what you could have them look like I have no clue ... you could say that they are real big to collect solar/cosmic radiation ( though they need not be), but Pluto is not that big. I would stay away from the normal SiFi, human in a jumpers, look. They would have some type of adaptation for ice because that may be the main think that they walk on (maybe hard shell like pads on the bottom of their feet/foot that shaped like a cheese grater...keep them from slipping?)
heck, they could melt in earth type temps. (Disad-Susceptibility: Temps in range X up )
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I just read your post again, I think I went a bit off topic from what you were asking for... sorry . I'll think about it and if I come up with something I'll repost.
Did you want game mech. as well?