Honestly, I suspect Surtur is a lot sharper than even demons tend to think of him being. I strongly suspect the Castellan of working under different constraints than Akio realizes--especially that her ultimate loyalty doesn't lie with him. Not that she has the slightest intention of interfering with him, but she may be reporting to one of Surtur's subordinates on the side.

Also, come to think of it, she looks demonic and reasonably attractive, but we got no description beyond that. I wonder if she's another plant from Midgard? There's a small but nonzero chance she answers to Illyana, maybe? Not as if Akio would recognize the accent.

------
Fearless creatures, we all learn to fight the Reaper
Can't defeat Her, so instead I'll have to be Her