Hilda is the bodyguard of Claude, and as such cannot be recruited from the Golden Deer House while you are in the Black Eagles House. However, if you are in the Blue Lions House, the restriction appears to be lifted, and Hilda can be recruited.

Why is Hilda not recruitable?

Hilda is the spoiled, rich daughter of a noble. She joins the Golden Deer house and acts like Claude's retainer. If you didn't pick the Golden Deer house at the beginning of the game, it appeared that you couldn't recruit her.

Is Hubert the death knight?

Verdant Wind

Hubert is among the forces joining Edelgard during the three-way battle at Gronder Field. ... Hubert makes a final stand in Enbarr, leading the Empire's forces and the Death Knight to defend the city.