Ran-Mao (藍猫, Ranmao; Chinese Pinyin: Lán-Māo) is Lau's personal assassin.
Ran-Mao has large, golden eyes and jet-black hair with thick, square bangs. Her hair is arranged in long braids and cat ear-like buns, accompanied by a pink peony and a golden tassel on the left bun. She wears a short, black and lavender cheongsam, that is decorated with dark-pink petals, and a blue jacket over it. Additionally, she dresses in thigh-high tights, black ballet shoes, and gold anklets with bells.
When acting as an assassin, Ran-Mao carries two chuí or meteor weapons. They are gold, blue, and maroon in color, and each chuí has a blue tassel on the top. They are composed of very heavy material, as a single swing of them is capable of breaking pavement.
Ran-Mao is cold, emotionless, and taciturn, as she hardly ever engages in a conversation. However, she is deeply loyal to Lau, and acts only when instructed to by him. Though she is technically not related to Lau, he claims she is his sister, since she has known him since they were very young.
Ran-Mao possesses some flirtatious tendencies, and usually under Lau's orders, she would attempt to use her sexuality to attract someone's attention. Before, at one of Ciel Phantomhive's parties, she had triflingly courted an important guest; Lau attributes her flirtatious behavior to the fact that she is a "pampered child."
Indian Butler Arc
Upon Ciel Phantomhive and Sebastian Michaelis' arrival at Lau's opium den, Ran-Mao is seen sitting on Lau's lap, along with several other women. She observes in silence at Lau's exchange with the guests.
Later, Lau decides to take Ran-Mao to the curry fair as a treat, which surprises Ciel due to the fact that he could nonchalantly have one of his girls in front of his clients. Lau claims that Ran-Mao is his sister, although they are not related by blood. Throughout the competition, she silently spectates the event, while standing in Lau's arms.
Afterward, Ran-Mao and Lau approach an upset Harold West Jeb and his wife Mina, who are plotting to try another scandalous act. Lau says that while Ciel would not take out the "small fries," he has to exterminate the bad rats infesting his city; thus, he keeps a "cat," referring to Ran-Mao. She swings one of her clubs onto the ground, with enough force to crack the cement. Harold and Mina scream, and they are, implicitly, killed by Ran-Mao.
Phantomhive Manor Murders Arc
Ran-Mao and Lau are invited to a party thrown by Ciel Phantomhive. She stands quietly while Lau makes small talk with Arthur Conan Doyle, and when later introduced to Georg von Siemens, she immediately clings to him. Lau apologizes in her place, attributing her bold action to her supposedly pampered nature.
In the midst of the festivities, Ran-Mao and the other guests hear a loud scream from Georg. They rush to his bedroom; inside, they discover, to their unanimous horror, that Georg has been killed. They contemplate about the details of the murder, and Lau suggests that Georg's death was a locked room murder. Everyone is, then, asked for his or her alibi. When it is Lau's turn, he says that he and Ran-Mao were drinking in the lounge with Karl Woodley. Ciel is the only one without an alibi, and thus, Arthur is told to watch over him in his confinement. Afterward, Lau and the other guests are escorted to their respective rooms.
The next morning, Ran-Mao and the other guests discover the brutal murder of Sebastian Michaelis. Eventually, they resolve to move Sebastian's body to the cellar for the meanwhile, and plan to continue the topic over breakfast.
Later, at the breakfast table, they realize that Patrick Phelps is absent. They head to Ciel's bedroom, where Patrick was assigned to stay in. When they find that the door is locked, Charles Grey swiftly cuts it down. They hurry inside, and discover Patrick's corpse. While everyone else is concentrating on Patrick, Ran-Mao and Lau rummage through Ciel's wardrobe, much to Ciel's annoyance. Soon after, they have dessert at the drawing room, where Ran-Mao silently observes as the other guests discuss the three murders. Ultimately, the group allows Arthur to dictate their actions from that time on, as through deductive reasoning, he is the only one who could not have committed any of the crimes. They decide to investigate Sebastian's body in the cellar, but Lau asks to stay behind to enjoy tea with Ran-Mao. Subsequently, Arthur explains to all the guests that their rooms and belongings must be checked, and they give their consent. Ran-Mao and Mey-Rin search Irene Diaz's room. A distressed Mey-Rin notes that Ran-Mao tends to leave things open after she is finished looking through them. She takes the liberty to sift through Irene's trunks as well, and puts a pair of her undergarments on her head, much to Irene's and Mey-Rin's alarm.
In due course, the group gathers at the drawing room once again. Soon after Lau suggests the existence of a "thirteenth person," Jeremy Rathbone appears. After they verify that Jeremy is not involved in the murders, Jeremy is allowed to lead the investigation. For dinner, the group eats a scrumptious steak meat the Phantomhive servants have prepared. Lau presses for the truth behind the incidents, but Jeremy advises them to not be hasty.
In order to catch Patrick's killer, Ran-Mao agrees to be used as a decoy dressed in Ciel's clothes. Upon Jeremy's call, Ran-Mao kicks off the murderer, and Charles Grey steps in to kill it, but Jeremy stops him. They later reveal it to the other occupants of the Phantomhive Manor that Patrick's murderer is an African snake, who has aimed to kill Ciel based on his scent; therefore, when Ran-Mao was dressed in Ciel's clothes, it came directly to her. Ran-Mao remarks that his clothes are tight on her. Lau comments that should Ciel have died, they would have been in trouble. She agrees, saying she is glad Ciel is safe, and hugs him tightly; Ciel begs her to at least put on proper clothing, and manages to escape from her grasp.
Afterward, Jeremy shows enough evidence supporting the idea that Karl Woodley is the murderer. Subsequently, Karl is detained, and the rest of them celebrate the solving of the case. Ran-Mao and the other guests later leave Ciel's manor.
Luxury Liner Arc
One morning, Ran-Mao accompanies Lau, as he goes to visit Ciel Phantomhive at the Phantomhive Manor. She is served food along with Lau, and she leaves with him when he is done discussing the questionable nature of the Karnstein Hospital with Ciel.
Public School Arc
Sebastian Michaelis asks Ran-Mao and Lau to attend Weston College's annual cricket tournament, along with Lau's collection of women, so that said women can distract the other cricket players for Ciel Phantomhive's sake. As a result, Ran-Mao, Lau, and the other women arrive at Weston College on June 4th, the day of the tournament. There, Ran-Mao holds up a betting board while Lau attempts to persuade other spectators of the tournament to place bets on whether Red House or Blue House will win the match.
Later, Ran-Mao sits on Lau's lap, and she and the other women flaunt their bare legs, which distracts the Green House players, as planned. After a certain informer reports them to the officials, Ran-Mao, Lau, and the others are forcibly taken away from the venue. Despite this, they are later able to witness Ciel's victory and the birth of the second "Miracle of Sapphires."
- According to the official Kuroshitsuji's character popularity poll, Ran-Mao is the twentieth most popular character in the series, with 74 votes.