Lang-Lang Hsu, also known as Ran Ran, is a supporting antagonist in the anime Steam Detectives. She is the younger sister of Ling Ling, the daughter of the brilliant Dr. Hsu, and the primary criminal partner of the masked thief Le Bled. In contrast to her sister's role as an assistant to the detective Narutaki, Lang-Lang operates as a nurse and occasional caretaker, though she uses these skills in the service of criminal schemes. She wears black clothing as a self-imposed sign of mourning for the patients she has killed in order to end their suffering, a practice that reveals a deeply conflicted nature.

Lang-Lang possesses a brilliant and calculating mind, driven by a desire for revenge and destruction. She is determined, intelligent, and versatile, with a broader knowledge of the world and a far greater willingness to commit morally questionable acts than her sister. Her primary motivations include supporting Le Bled in his thefts, corrupting Ling Ling's innocent worldview, and humiliating or killing Narutaki. However, beneath her hardened exterior, she harbors moments of doubt. Narutaki's genuine kindness occasionally stirs long-buried positive feelings within her, causing her to waver in her commitment to destroying him. Similarly, she is reluctant to directly harm her sister, preferring to find other ways to remove Ling Ling from her path.

In the story, Lang-Lang serves as a recurring antagonist and a foil to both Ling Ling and Narutaki. Her partnership with Le Bled is central to her role; she looks after him, worries that he pushes himself too far, and cares for him to the best of her ability. Their relationship is hinted to be romantic, and she is fiercely loyal to his cause. She inherited the design for Dr. Hsu's last Megamaton, Hiroh, which she had built specifically for Le Bled's use. This technical inheritance showcases her intelligence and her connection to her father's legacy, even as she uses it for crime.

Her development is most notable in the episode "Tears of a Black Angel", where she takes a critically ill girl on a trip to the beach before a risky surgery. In that moment, Lang-Lang acts out of compassion rather than malice, demonstrating that her character is not purely villainous but torn between her vengeful goals and a buried sense of empathy. Over the course of the series, her resolve is tested, and she occasionally shows reluctance to fully commit to Le Bled's more extreme plans, particularly those involving Narutaki. Her notable abilities include her nursing expertise, her inheritance and knowledge of Megamaton design, and her sharp deductive mind, which makes her a formidable opponent for the Steam Detectives.