Masayo Aoshima (Aya Hisakawa) is the villainess of "The Locked Bathroom Murder Case", episodes 5.15 and 5.16 of Case Closed (airdates October 26 and November 2, 1998). She was a bank employee and the older sister of Mina Aoshima, whom was also her housemate.
Masayo's slight backstory also revealed that Mina had a long history of "copying" her sister, even to the point of buying the same furniture and liking the same music as Masayo. While she initially found her behavior cute, Masayo eventually tired of her sister's behavior and her feelings came to a head when she learned that Mina had begun dating and became engaged to Kenji, Masayo's ex-boyfriend.
Enraged, Masayo plotted to kill her younger sister and stage her death as a suicide. As Mina was helping Masayo pack up, as Mina wanted Kenji to move into the house, Masayo asked Mina to tear off strips of packing tape she needed to seal the boxes of her stuff. After that, Masayo gave Mina a can of beer drugged with sleeping pills and took her incapacitated sister to the bathroom, where she planned to use cleaning chemicals Mina had recently bought to gas her sister to death. But when she saw the chemicals wouldn't produce a lethal amount of gas, the evil Masayo filled the bathtub and cut her sister's wrist, placing the wound in the water so she would quickly bleed to death. Afterwards, Masayo used the packing tape Mina had pulled off to seal the door and window to make it appear that Mina had sealed them in an initial suicide attempt via lethal gas.
To establish an alibi and fully make it appear that Mina had sealed the bathroom herself, Masayo intended to use Kenji as a witness when he came to take her and Mina to a Yoko Okino concert. But when Kenji cancelled due to being sick, Masayo took up detective Kogoro Mouri's plea to take Kenji's ticket in his place, using him, his daughter Ran, and Conan Edogawa as witnesses to Mina's "suicide". Masayo had not fully sealed the bathroom door, but instead acted as though it were taped shut and opened the door just as the four were preparing to break it down manually.
Masayo's deceptions initially worked, with Masayo giving off a satisfied laugh as she went into her bedroom to find the ticket for Kogoro. But Conan, having become suspicious of Masayo, investigated and eventually deduced the truth. When Conan impersonated Kogoro and confronted her, Masayo attempted to feign innocence and claimed that "Kogoro" had no evidence to her guilt, but Conan in turn revealed the piece of evidence revealing her guilt: a spot of blood on her front door above the peephole. Masayo had gotten some blood on her Yoko Okino headband, and transferred it to the door when Kenji came to the house during her murderous plot to check on Mina.
Masayo again tried to deny the claim, but realized upon looking in a mirror that the blood was still on her headband, sealing her fate. Masayo confessed to killing her sister, expressing her resentment at Mina for copying her and (in her mind) stealing Kenji from her, who revealed that it had been him and not Mina to start their relationship, as Mina cared too much about her sister to date her boyfriend. After Conan also revealed that Mina's purchase of two cleaners that wouldn't become toxic when combined proved Mina's love for her sister, Masayo broke down in tears and cradled her sister's body while begging her to wake up. Masayo was presumably arrested offscreen for Mina's murder.