From the Kingdom of Death and his Eternal Bride, come two most singular princesses, ladies in waiting from vassal kingdoms:
Elizabeth Tenebra, the first daughter of Kain, who killed her father, but still keeps up appearances as though the Demon Lord is still alive, because she believes the court would neither respect nor value her opinions on her own.
She is the Oldest Monster.
The first Vampire.
She's crazy strong and crazy hungry, though eons more disciplined than her "canonical" self. She prefers to avoid feeding on sentients if she can
help it, but if pressed will admit she likes a good scrap.
She is capable of shape changing but finds it "undignified."
Her senses are, though, sharper than ever. She can feel and see and hear and smell everything in almost 100 miles of herself.
She is still somewhat vulnerable to Death magic, but even if her soul is reaped, she will merely make her way back to hell.
She is still a child at heart, with all the naïveté that entails. She's just playing Princess instead of Monster.
Olivia Nihil, from the Entropic Lands, the kingdom after the end of all things. A vassal state only in the sense that it pays tithes. OBLIVION existed before the paltry ending of small biological life was but a twinkle in some cosmic eye. But diplomacy is important, so it sent it's one human daughter-self to wait upon the more conquest-happy Death.
She is completely indestructible through an unusual mechanism: any matter or energy that would so much as singe her clothes is unmade, consumed by endless nothing. She also has a divine awareness that just so happens to be a bit too imprecise to actually detect anything less powerful than, say, a major Duke of Hell.
There is also the small matter of her being capable of consuming at the very least the mass of a large city.
She can also walk between worlds and move unseen by folding space. She can use this ability for Hammerspace purposes as well.
As for weaknesses, she is a scared little thing, who tries to keep kid gloves on whenever possible. She moves like a bull that knows the value of a china shop.
And though it may be impossible to annihilate Annihilation itself, she can be bound outside of her human body, functionally identical to death.
After all, she's already accepted one gilded cage.