The Outsider Walker named "He Who Walks Beside" is commonly known as Nemesis. He's a specific Outsider, who specializes in infiltration. I rather think Demonreach was setup to deny him access to hosts he'd otherwise infected, and therefore free to deny their natures.
I know that, but was it a Walker who infested Cat Sith? No Walker or Outsider was mentioned there, only Nemeses.
Harry does say he thinks Shark Face got to him and that is how he got infected. But that is all, no Outsiders.. Harry further says; on page 427 of Cold Days.
"Like Mab wouldn't have figured it out," I said. "Like she did when you infected Lea."
Or later page 503 Cold Days
"It was the knife," Mab said.
"Knife?"
"Morgana's athame," Mab said in a neutral tone--but her eyes were far away. "The one given to her by the Red Court at Bianca's masquerade. That is how Leanansidhe was tainted--and your godmother spread it to Maeve before I could set it right."
So, no mention of Lea or Maeve being possessed by a Walker or Outsider, merely that both got infected by contact with Morgana's athame which was tainted. Some how Sharkface an Outsider
managed to get Cat Sith infected, however he never possessed Cat Sith. So Justine is infected and possessed?
My theory, just as the plan was to infest the more important members of the Winter Court to spy and undermine.. The same plan was to undermine the White Court. Bianca wasn't blind, she may have observed the closeness between Justine and Thomas, so just as the knife was used to get to the Winter Court, Justine could eventually be used to get to the White Court.
