What should I do with the knight?

Hint 1 of 11:

You want him to open the stone door in the floor of the Church.

Hint 2 of 11:

He seems upset.

Hint 3 of 11:

Over Diana, actually.

Hint 4 of 11:

He won't help you with the door, until he can feel better about Diana.

Hint 5 of 11:

You want him to put the marble rose in her hair.

Hint 6 of 11:

He can not hear you, so you'll need to show him something that tells
him what you want him to do.

Hint 7 of 11:

Something you got in Alexandria will help.