... The first character who we can reasonably guess was nemfected is Aurora. We don't know when Nemesis got to her, just that it was before the events in Summer Knight and because Elaine was spending time in the Summer Court, she makes a pretty good suspect. The problem is we do not have any information about anyone else who may have been visiting Summer or had contact with Aurora. If you go back and reread the scene in Summer Knight when Harry brought a wounded Elaine to be healed by Aurora, there were one or two other humans who were working with some of the Summer Fae on a couple of artistic endeavors. They were about as minor as characters can get. They didn't have any lines of dialogue and weren't named, but there very presence tells us that some other mortal could have visited Summer and infected Aurora. How it was done is a mystery.
I don't think you can rely upon those minor characters being human. There was all sorts of faerie glamour in play.
But I'm pretty sure Elaine herself was the Nemvector that got Aurora.
She went from Justin's enthrallment to Aurora's bargain, and likely carried Nemesis with her.
... Justine is a real mystery. I doubt she was contaminated because Nemesis jumped from Maeve when Murphy shot the ice princess in the head. Justine wasn't acting like herself for a long time before Cold Days. In the short story Even Hand she was acting like a top flight secret agent when she chose to go to Marcone's HQ in order to evade the Fomor lord who was chasing her. That story took place between the events in Turn Coat and Changes, but I suspect Nemesis got to her much earlier than that. Just before the duel in the Raith Deeps in White Night, Justine surprised Harry by the confident and collected way she was able to describe the events taking place and rivalry between Skavis and Malvora. It was almost like she wasn't the real Justine.
But when Justine got the Nemesis bug isn't nearly important than how she got it. That is, if I'm right that it didn't happen on Demonreach near the end of Cold Days. Because, if I am right, then Nemesis got to Justine while she was under Lara's protection. Justine would have been on the grounds of House Raith or at Thomas' apartment most of that time. Here's a possible explanation how it could have happened. Maybe it happened when she did her secret agent thing offscreen before the start of Even Hand. I believe she infiltrated a Fomor base. She would have been vulnerable then. Maybe she wasn't successful and was captured but Nemesis was present in someone or something else and saw an opportunity in Justine by making it look like she had succeeded. It would explain her demeanor with Marcone. I haven't reread that story in a long time, but I recall that Justine came across as very calculating in her decisions and actions. Perhaps Justine had told Harry the truth in White Night when she said medication had made her a more stable person.
I think Justine got infected by something in Papa Raith's collection.
We know Raith père had Outsider-magic protecting him. We know he also had an Outsider-summoning ritual. IIRC, there's WoJ saying he had become aware of the Starborn cycle, which is obviously linked to Outsider action; and he was working to become a power-player in the big battles to come. He had amassed a huge library, trying to get himself up to speed. It seems pretty likely that a relative novice, collecting like mad and scrambling for power, will make some bad decisions in this arena and unwittingly collect some very dangerous items.
Plus, it'd make good storytelling sense for Harry to get access to such a trove; and lo! his new fiance is heir to that collection.
I figure Lara had tapped poor, vague Justine to help her with sorting the collection (the odds of Justine being able to focus enough to recall anything useful, or report to others, would have seemed minimal). But something in there spoke to her... offered her the chance to help Thomas, the chance to
be with Thomas again.
What else can you think of that we know or can reasonably guess about Nemesis? I suspect there is at least one other character who is nemfected. Is there anyone else you think could be a Nemesis agent?
Nemesis wants to get close to the power-players...
It likely has someone near to Marcone... Demeter/Beckitt? She has already shown she is willing to make poor deals for power/revenge. Marcone's enforcer, Childs? I don't think we have much evidence, but I think *someone* close to Marcone is Nemfected.
Likewise Odin. He's a much harder target, but even more valuable to Nemesis. One of the Einherjaren, or even one of the Valkyries? Again, I think we don't have any evidence, nothing shown on-screen.
I'm sure there's another Nemesis agent within the White Council. I'm thinking someone high up amongst the Wardens, but possibly one of the Senior Council. Luccio looks likely to me... relatively-recent psychic wound, struggling emotionally, trying to get/recover power, highly-trusted... pretty much perfect, in fact. Plus: Luccio would hurt Harry
way more than if it were one of the minor named Wardens.
Unspotted agents seem likely in any given Faerie court. Winter (as the primary Outsider-foe) would be most-valuable to Nemesis, but Summer can be almost as good; the Erlking's court, too. As above, I don't think Jim has written any hints/clues/evidence onscreen for any of these.
I expect others, but none spring to mind at the moment.