Case Closed: The Raven Chaser

A man driving down a hillside road realizes that his car's brakes have failed. The car crashes into a toll booth, killing the man. He leaves a dying message: "Tanabata kyo." A Mahjong tile next to his body links this case to six others in Tokyo, Kanagawa, Shizuoka, Nagano, and other Japanese prefectures. Because of the Mahjong tiles left beside each victim, the police conclude that the same person or organization committed these murders. The police across Japan unite to solve the serial murder case.
After a police conference about this case, Conan discovers a police officer entering a black Porsche 356A owned by Gin. He concludes that a Black Organization member disguised as an officer has infiltrated the meeting. On Tanabata, Conan corners Vermouth in the underground parking lot of a shopping mall. She reveals that the Black Organization needs a memory chip, which the murderer took, and a new member, "Irish".
Conan employs the help of Heiji Hattori to solve the case by following the dying message lead. He and Heiji find out that two years ago, an accidental fire in a Kyoto hotel killed a young woman named Nanako Honjou. The elevator could only hold seven, but eight needed to evacuate. Nanako was not on the last elevator and did not escape.
Conan tracks down her neighbor, Shun Sawamura, and asks him some questions. He finds out that Nanako and her boyfriend, Kousuke Mizutani, often went stargazing together. Conan realizes that the crime scenes are ordered like the star constellation Ursa Major, making the last crime scene Shiba Park. The police also figure out the pattern and leave for Tokyo Tower.
Conan confronts Mizutani, who is about to commit suicide in Nanako's memory and to avoid arrest. Mizutani thought one of the seven had pushed Nanako out, but Nanako's brother, Kazuki Honjou, shows up and testifies that he had overheard two survivors say that Nanako voluntarily stepped out. Conan reveals that the murderer is Kazuki because Mizutani would not murder to stain the memory of Nanako. He tells Mizutani that the survivors appreciated Nanako's sacrifice and sent flowers to her memorial. Kazuki lied to manipulate Mizutani into avenging Nanako but did it himself after realizing that Mizutani would not. Mizutani realizes that Nanako would not have wanted him to commit suicide and returns the victims' belongings to Kazuki, who would have used the evidence to convict Mizutani. Kazuki threatens Conan and Mizutani with a knife. Matsumoto appears and shoots the knife out of Kazuki's hands.
Conan realizes that Matsumoto is Irish in disguise. Irish claims that he does not want to kill Conan. He says he discovered Conan's true identity, having matched his and Shinichi's fingerprints, and wants to turn him in to the "boss" alive. The "boss" would know of Gin's mistake and punish Gin, satisfying Irish's revenge against him for killing his friend Pisco. Conan wants the memory chip, but Irish refuses to hand it over. Ran arrives at Tokyo Tower and finds Irish still disguised as Matsumoto. She uses karate, but a kick to his face tears off part of his mask. Taking advantage of her surprise, Irish overpowers Ran.
Irish and Conan move outside near the top, and a helicopter with Gin, Vodka, Korn, and Chianti arrives. Irish shows them the chip, and Gin orders Chianti to shoot Irish and the chip. The bullet seriously injured Irish. Conan tries to drag him to safety, but Gin notices Conan and tries to shoot him. Irish shields Conan and dies.
The Black Organization attacks with a machine gun, damaging the Tokyo Tower. Cornered, Conan escapes by slinging a skylight with his expanding suspenders at the helicopter, damaging the engine. The Black Organization manages to fly away, but the helicopter crashes. Conan later discovers that the members escaped the helicopter before it crashed and vows to take down the organization once and for all.
Professor Agasa and the Detective Boys free the real Matsumoto from a small house in Beika Woods with the help of Officer Satou and Takagi.

Question: who finds out that Nanako and her boyfriend, Kousuke Mizutani?
Answer: Conan; Conan