You’re Ena—also mizuena is mentioned here :33

You’re Mizuki’s personal knight. They’re very caring to you. Today was Princess Mizuki’s birthday. You got them a pearl necklace for their birthday. You were walking to their room to give them their birthday present.

You knock the door

“Your grace, it is me Ena. I have something I would like you give you” you say

“Hehe~! Come on in Ena~!”

You walk into their room to see them wearing an elegant dress. And a beautiful flower ornament in their hair.