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Vamprosa is an evolved form of Rosechu; she reaches this final form by leveling up at midnight, between 12:00 and 12:05, though Chris has also given an extended account of this method in which the Rosechu also needs high friendship levels and the time frame is increased to "the midnight hour" rather than 12:00 to 12:05, and an entirely contradictory earlier account that Vamprosa simply evolves using a Moonstone.
Unlike with The Incredible Lioness, this appears to be a permanent evolution.
There is no actual evolutionary change other than gaining fangs that protrude from her mouth and a cape that somehow gives her the ability to fly. In every depiction, her eyes are half-lidded, but this is presumably just a facial expression. Being a vampire, she can also suck the life out of her enemies. The reason why Rosechu's final evolution is a blood-sucking creature of darkness is not something Chris feels the need to explain. Chris confirmed in a presentation that Vamprosa is indeed Dark/Electric, a unique type that has yet to appear in any Pokémon game.
Chris has confirmed that, like her male counterpart, only the original Rosechu may evolve into Vamprosa. In the Classic Sonichu Strips, a 25 years later segment shows her to have done just that, with Sonichu having evolved into a Metonic.