I know I am 99.99% wrong but what if the boy Fiona saved was MC. She let some family adopt the boy while he was 2, so MC would not remember that any way. Also, if MC is not a human, it would make sense if she entrusted him to someone from Templars because they are aware of other races. Maybe she asked help and MC's parents took a notice he was important so they decided to adopt him and raise him as their own.
This was mentioned long ago, and while possible, here's counter-question: Why did she enter his life now?
Fiona is capable of following Templars to meeting with Alfar in Neverwhere without being seen.
She's not so clumsy to bump into random 9-5 salaryman twice.
Which begs another question: Did she know MCs father will be kidnapped and decided to enter MCs life more openly?
Finally, and I'm serious here, could MC be far descendant of Medb's adopted human brother?
Could he be last in that bloodline, and that's why the sword is so compatible with him?
If she's so linked to MC, how much of what she's hiding is about him? How much will be things he should have been told? Will she also be one of the girls that withheld vital information from MC?
Some said, and I'm with them, that Fiona obviously hides some things from MC and we're fine with it because they literally just met few days ago.
But will it be the same if it turns out to be false, and she observed him his whole life?
...Kaija and family...
If so, we really can't blame Kaija or the parents or Fiona like we can't blame Kenobi or Dumbledore. Now, the third party either figured it out MC is the chosen one or they took desperate measures to prevent Templar's plan so they took a leap of faith by attacking. If this is true, there must be members of third party in Templar ranks as well as alfar. Sending Fin to a mission where she will die is an exactly the type of a thing a inside man would do. It really caused MC's exile and any means of peace. I think there are some hints and clues that this is a possibility. It fits like a glove to chosen one Trope. An orphan saved, adopted, trained then become the savior of the world.
Still disagree on statement about blame.
No matter what grooming, no matter what the intention, you don't intentionally ruin someone's life because you want them to be either safe, or something of higher purpose or whatever.
It's still taking control of someone's life
for your own gain because you believe you know what's best for them, and take that choice away from them.
Kaija is smart, adult, thinking person. She wasn't closed in 4 walls and brainwashed to that level.
She was definitely brainwashed to believe Templar agenda, but with full access to all wonders our world has to offer (internet, news, books etc.). She had to know better.
Parents? They are no parents of this MC. They are garbage human beings, whatever their reasoning. Parents that not only allow, but support and agree with, fake relationship which will mentally scar their son, aren't worthy of having children. They had plenty of outs there, some very simple, and chose to take none because they were happy with current arrangement.
If however, you mean they can't be blamed for hiding Neverwhere stuff from him, then I withdraw paragraph above. I don't blame them for hiding anything. That part I mostly support and is not reason why I hate them.