You're right, I did consider this as well... But then the following is also true:
Devil's advocate, I'd argue that the MC was a special case, being the son. Still, Kaija knew her secret and activities put stress on her relationship with the MC. It's clear at the start of the game that neither of them are happy with how things are going. We've also met a few people who either resigned or took on different roles, so there was a path for Kaija, if she wanted to prioritize her relationship over her activities.
Let's be fair, how effective is a bodyguard when she's away on missions more often than not?
I agree about the bodyguard part, especially since Kaija will only receive orders to protect the person she's bodyguarding if she has a bad relationship with him. The Doylist explanation is obvious, but the Watsonian explanation is tortured at best.
That said, I don't think it's hard to understand why Kaija might not want to retire from the Templars entirely. We've seen a pretty poor showing from them thus far, but they presumably must be capable of doing
something well if the Elves went to the effort of trying to disband them. It's entirely possible Kaija can legitimately believe working for them is a net good to the world, in which case I can accept her divided loyalties. Sometimes duty requires us to do things we'd rather not, but that's just life. It's not like she's married to the MC, either. Expecting her to undermine a
potential greater good and throw away her whole life (a life she's clearly
very good at) just to tell the MC something his own parents insist on hiding from him is, IMHO, unreasonable.
But that's me. I tend to be somewhat forgiving of the 'lawful' end of the spectrum in general.
