Megalomaniacal blondes who despise humanity, are over a century old, and were responsible for creating the main antagonistic force of the series.
They absorbed other beings in order to grow more powerful. As an added bonus, their main enemy is a red-clad blonde who holds a legendary reputation, is a master of his craft, and wields a prosthetic arm that doubles as a weapon.
They also have a bunch of inhuman cronies.
They are blood related to protag in some way. (Knives being Vash's brother, and Father's body being a copy of Van Hohenheim and Edward)
They have multiple schemes in which one plan involves their twin in the destruction of an entire city.
And they have some sort of secret big plan where they have this powerful form (Which is related to divine entities) but were defeated after exhausting all their energy.