I think one of the most interesting aspects of Bernkastel’s character is her origin and how it relates to Higurashi.
It’s heavily implied that (lots of Higurashi spoilers) Bern is the result of an unfinished game between Featherine and Lambdadelta. The events of Higurashi are just a few fragments from a game board that Featherine - through her piece, Hanyuu - has abandoned, leaving the game in a state of logic error. This leftover piece is simply an observer, unable to act on events directly. Through some sort of miracle a separate, insignificant piece on the gameboard was able to bring about the conclusion of the game after travelling through thousands of fragments to reach an impossibly ideal scenario - Rika. She even convinces Featherine’s abandoned piece to join the fight, and manages to end the game. This almost-endless struggle gave birth to a new witch, who takes on the suffering of Rika’s thousands of bad ends in her place, so that the piece can experience a happy ending. Bernkastel actually appears in a hidden scene in Higurashi (or in the anime, if you watched it) which I believe further confirms that these are more than simple parallels but are in fact, the same character, in a sense.
I think that Bernkastel’s status as Ikuko’s former cat represents (beyond a reference to Rika’s punishment game outfits!) her role as an ascended piece on Featherine’s side of the board, hence Ikuko’s previous ownership of her. The miracle parallels are obvious, as is her relationship with Lambdadelta - who is equally responsible for Bern’s suffering and existence through her role on the previous game board, represented by her piece, Takano.
I’d like to draw parallels between Lambda and Takano’s “God”, but that’s another story…
I wouldn’t call her cruel, but rather empty. Her personality matches up with “dark” Rika, on the occasions where the despair of too many doomed timeliness catches up with her. Her cynical attitude only shows a lack of empathy, in my opinion. The characters that she shares most emotion with are Lambda (who as previously explained is the cause of Bernkastel’s despair, and yet the very reason she exists. As such, this relationship is very much a love-hate ordeal, although my words don’t quite do it justice) and Battler ( who has also had that similar experience of a logic error) and both are justified.
That said, I don’t think she’s “faking it”. She schemes and plots to incite despair in the other members of the cast deliberately, but is she really capable of anything else? For a witch with nothing but suffering as her companion, I doubt she has an alternative to this cruelty.