I was gunna say that too. A she dress up as a he but the princess doesnt know. But as the marriage started he used revealed his secret just to portect the princess. But the princess just doesnt know...or so i think...
Woah...
So the princess doesn't know that Mitsu is biologically a female...technically that makes this shoujo ai, right?
Or is Mitsu a trans male and not just choosing to dress as a male for other reasons but feel they are themselves a male? So that pairing of Mitsu and the princess is straight but with Nobunaga it's gay? A bit confusing but very interesting.
SPOILER:
So, he's really a she, and not the other way around?