But what about the Leanansidhe? She gave the athame to Maeve, which got Maeve infected, if I remember correctly.
Later she had to be contained by Mab in the icicle in Arctis Tor to get healed from the infection with Nemesis.
Does that mean, Lea was a vessel for Beside, too and he just hopped into Maeve, after he made Lea arrange that. So Lea was just a means of transport for Beside or was she just influenced by him through touching the athame and only Maeve was the real infected one? Or did it infect Lea, Mab healed her and only after that it had to flee into Maeve, which suggests, Maeve wasn't his original target?
But after the athame, which Lea received in Grave Peril, how did Beside get into Aurora? I have to reread.
Or was Aurora even possessed by Beside or just influenced by him, while he possessed Maeve?
I'm a bit overwhelmed by all the implications). And since when was Beside in Justine? Did he jump hosts out and back again? Was he in Justine since the end of Grave Peril and did he just influence the minds of the Ladies and Lea and Cat Sith?
WAG:
Without having reread everything from Grave Peril on, I can only guess.
My guess is: Nemesis/Beside chooses a main host. From this main host's body he can jump to other hosts and back to the main host. For changing hosts he needs a tool, for example the athame from GP. All the jumped-in hosts remain under his control and he can jump back to the main host any time. Once he has jumped into one host by tool, he can jump back there any time if there is need or influence the interim host's mind from the main host's body.
Justine is the main host, because she is emotionally instable.
What if she wasn't catatonic alone because of Thomas almost killing her (in Blood Rites?), but because he was scheming in another body (Lea?) during that time, because Justine was weakened too much to be of use.
Or: Beside can jump into emotionally instable persons very easily, by skin to skin contact from another host, but for the stronger potential hosts he needs a tool like the athame. This made him able to inhabit Lea.
Edit: Thomas is constantly fighting his Hunger, so he is practised in fighting off possession of other kinds. So not an easy potential host.