I have the same difficulty, and this is one of my specialist subjects too. Since there is clearly an implication that "?" is unique I am scratching my head to find anything common to the 5 members of Group 2. Putting it aside for a while might help.
I now have a full grid and the two groups - and, apparently like everyone else, no idea what to do next! I'll keep working, keep staring,keep thinking...
My best guess is that all the 'characters', i.e. letters, in Groups 1 and 2 are the same (i.e. as groups they are anagrams of each other). I've tried removing all the Group 2 letters from Group 1 though and what's left doesn't seem to give me anything that can be anagrammed into a further member!
Boreas, sorry - I think I assumed you got the same enlightenment as I did from Gasco's tip. Like you, I thought the very end of this puzzle was a bit uninspired - I thought 'is that it?' and was glad to be finished with it.
Yes, it was good.
But why would the unknown character belong to group 2 - other than just to satisfy the mechanics of the puzzle?
And did any unknown characters ever produce anything that would rank them with any of the names in either group?
Or have I missed the point?
Just having a grumble. Hope it doesn't sound like sour grapes.