Eiko Nakayama (Kaya Matsutani) is a minor villainess from "Stage Actress Murder Case", episode 3.13 of Case Closed (airdate July 14, 1997). She was an actress for a theater group, and the episode had her and her crew producing a movie with famous actress Shoko Oide as the leading lady.
As shown early in the episode, Shoko had quickly earned Eiko's ire with her self-centered and demanding attitude. To get back at Shoko, the enraged Eiko began sending threatening letters to the theater, stating that tragedy would strike the theater if the production wasn't cancelled.
Detective Kogoro Mouri was brought in by the theater to investigate, and it was soon after that Shoko was killed during the production via cyanide poisoning. Eiko was initially the first suspect due to her dislike of Shoko, and she was even further incriminated by Kogoro catching her as she was trying to hide the threatening letters she'd written and poison being found on the glass of water Shoko had taken a drink from.
Despite this, however, Eiko fiercely proclaimed innocence in Shoko's murder, and she was temporarily exonerated by the revelation that propman Junichi Tateoka had given her the glasses of water for Shoko's scene. Eiko was definitively proven innocent by the reveal that Shoko had been poisoned by her manager, Hitomi Sasaki. It was left unknown, however, if Eiko was arrested for making the threats against the theater.