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. Also, if Kaija's family somehow important in the Templars like generations of guards, it make sense they made sure Kaija becoming his guard. As far as we know, Kaija's training started around she met MC, so maybe she is purposefully trained to protect MC. I think that also make sense because when you put 2 kids together they would become friends. After that, I believe they hoped they fell for each other and maybe, without MC's or Kaija's knowledge, they were grooming MC for the grandmaster role while Kaija stand right next to him. We know MC's father was training him while not revealing what was his training for. Even Kaija mentioned you do know how to fight to MC. Also MC excelled whenever needed. On top of that, he picked up sword fighting rather quickly. I don't think none of it could happen without a foundation beforehand. It looks like they are still hiding secrets from MC and if it's about MC's past, it's not easy to reveal such a thing. Mainly, it lifts MC's protection. When you think about it, after learning about Neverwhere he found himself at there almost immediately. Maybe they are believing MC to be the chosen one, the one who would bring peace to Neverwhere. If so, not telling MC's past, Neverwhere etc. is not for protecting MC by keeping him in the dark but hiding him from harms way like Harry Potter or Anakin or any other chosen one genre. We still don't know why or who attacked MC. It was clear elves did not do it. Maybe third party is not a race but a group that trying to prevent the chosen one for protecting status quo because time and time it's been mentioned there are many profiting from the Neverwhere's chaotic nature. Peace would be bad for business.
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.
What do you guys think? I think it needs tweaking here and there but it sounds like a legit theory. My only problem with that, that Ceolag's writing is too unique to fit to a Trope that used rather quite frequently.