Yeah this is going touch upon some spoilers for the game if you're still new-ish to it.
Convenient TL;DR - short woman (think like, 5 feet at the most) with lots of power who acts generally bored with mortals, etc etc. Will make a snack out of a bunch of people. Perhaps let herself be one to those she finds interesting enough.
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As you've probaly guessed, Rosaria is way older than she appears, as a short unintimidating woman (at least until she opens her mouth), and is way more powerful compared to most Counters in the Counter:side universe - she doesn't even get out of that seat when fighting normally!
By normal she masquerades as just a very powerful woman with strong telekinesis that just sounds very full of herself, even if she can make most people regret underestimating her. For the very few that can get her to actually use some of her real strength though, in that third picture?
She's not even going be mad about it - she's bored enough that someone providing a challenge for her would be interesting, if it's not one of the few people she knows can do that like Hilde.
And as you might guess this lends itself quite well to consuming others around here - whether it's because they couldn't manage to beat her, or did the awful crime of boring her, a foolish mortal can make a great meal.
Abilities
Figured I should put this down so people have a rough idea of what to expect, in her default form where she's generally lounging on her throne:Being a Counter (sorta) - a tl;dr summary, Counters are people enhanced with superpowers, with general improvements like increased durability and fighting speed. Rosaria...isn't one per se, but she's close enough for people to think she is one.
Pyrokinesis - her bread and butter, basically. Whether it's small fireballs or giant beams of burning light, you will have a high chance of being made crispy.
Telekinesis - Not too strong but enough to let her float - and be a danger to people who think they're clever trying to attack her when she's floating in the air.
Demon's Flame - her sword. She rarely takes it out when she's lazing but it's a good way to punish people who've gotten too clever while not quite rousing her fighting spirit.
And of course, for those who do rouse it (e.g see Hilde when she's going all out), she will gladly step off that throne and go into that form depicted in the third image of the gallery, sword at the ready.
And you may just find out why she's a Demon Lord...