Rain Hasumi is a supporting character in the anime Valkyrie Drive: Mermaid. She is a resident of the artificial island known as Mermaid, where carriers of the Armed Virus are kept apart from the outside world. Unusually among the island's inhabitants, she belongs to neither of its main communities, the Veste castle and the villages. Instead, she lives a free and independent life alongside her partner, Lady J, and the two are known together as the duo Lady Lady. Rain has medium-length blonde hair and is almost always seen wearing sunglasses and a distinctive open leather rider suit.

Rain is notable among the island's residents because she possesses both kinds of Armed Virus abilities. She can act as a Liberator, and she can also serve as an Extar by transforming her own body into a cross-shaped cannon called the Disarmament Buster Arm. In that weapon form, she can use her signature attack, the Disarmament Buster, which forcibly cancels an Extar's transformation and returns the enemy to human form, although it has no effect on ordinary soldiers who are not Extars. She can also reverse the process and transform Lady J into a motorcycle, which she then rides into battle.

In the story, Rain first appears when the protagonists Mirei Shikishima and Mamori Tokonome are overwhelmed by Wärter soldiers. Rain kisses Mirei in order to trigger her own transformation into the Disarmament Buster Arm, breaking the weapon states of her opponents and allowing the Lady Lady pair to fight off Charlotte Scharsen and Kasumi Shigure. Her actions draw the attention of Wärter's governor, Akira Hiiragi, who notes that Lady Lady functions differently from other pairings on the island. Rain's motivations center on preserving her carefree way of life with Lady J, and she approaches the island's conflicts on her own terms, even placing bets with Lady J on the events happening around them.

Rain's defining relationship is with Lady J. The two share a deep bond of trust, both personally and in battle, and their seamless teamwork is the reason they are recognized as a capable pair. Her more casual interactions with the main characters reveal a playful and confident personality, but her loyalty to Lady J remains the constant guiding her choices. Over the course of the series, Rain does not undergo a major personal transformation; instead, she functions as a skilled wild card whose unusual dual abilities and outsider status make her a valuable ally to the protagonists while she keeps her independent life on Mermaid.