|“||Unwilling to listen or see anything else, you lost yourself in your own world and became incapable of seeing the things around you.||”|
–Raikou, episode 10
Raikōu Shimizu (清水 雷光, Shimizu Raikō) is a 20-year-old samurai of the Shimizu clan. His sword is the white gamon or Shirogamon (白我聞), the partner to his sister Raimei's sword, Kurogamon. He is voiced by Junichi Suwabe in Japanese and J. Michael Tatum in English.
When Raikou was fifteen, mutany within his family broke out against his mother, the leader of the Shimizu clan. His uncle Iroku disagreed with her ways, and wanted to take an active role in both worlds, the world of normal people and the hidden one of ninja's that existed within it. He wanted to use their ninja abilities to actively help people; while Raikou's mother thought that they should stay in the middle, and only intervene to help keep a natural balance. Iroku confronted Raikou's mother telling her to step down as the head of the family, but killed her and Raikou's father in a rage when she declined. Raikou had originally agreed with his uncle because of an incident when a friend of his was in trouble and died because his mother forbade him from helping her, but upon the murder of his parents, he took up his sword and used it in punishment; murdering Iroku for his crimes. This lead Raikou's killing of the majority of the Shimizu family as well; anyone who was not already dead from another family member's hands. The only other surviving member of the Shimizu family after this incident is Raikou's sister, Raimei , who was only ten at the time. Awoken from sleep, Raimei stumbled upon a burning and bloody building, a room full of her dead family members, and only Raikou standing alive with his sword. He took full responsibility for the deaths of everyone so Raimei would not have to bear the burden and could remember her family as good people; especially their uncle. Raikou then tells Raimei that he is going to go join the Grey Wolves. Believing that he killed their entire family in cold blood, a hatred for Raikou quickly grows in Raimei, making it easier for him to leave.
Raikou works as a punisher, cutting down those who betray the Kairōshū in the Wakachi with his partner, Gau, a boy whom he rescued from death three years ago. Upon his victims, Raikou places spider lilies with the intent of luring Raimei to a field of spider lilies so that he could confront his younger sister later on.
He chooses not to tell Raimei his motives behind the massacre to protect her innocence. This leads to a battle between the two, as Raimei finds it her job, as part of the Shimizu clan, to deliver judgement. In her eyes Raikou has soiled the Shimizu family name. She finds Raikou near where the Shimizu house once stood, the place where the red Spider Lillies grow. During this battle, Raikou cuts deep into Raimei and leaves her bleeding, and incapacitated in the rain, where he then leaves with her sword, Kurogamon. Later, he confides in his partner, and only real friend, Gau, after he is asked about his past. He tells Gau the whole truth, but makes him swear to secrecy, and makes it clear that there will have to be punishment if he doesn't keep his promise. However, Gau is deeply moved by the story and believes that Raikou and Raimei should be brother and sister, not enemies. Tracking Raimei down, Gau tells her what happened 5 years before, holding a kunai to his throat and betting his life that it's true. Later, after she gets out of the hospital, Raimei goes forward to meet Raikou for a rematch. In the field of Spider Lillies, Raikou gives Kurogamon back, but quickly disarms Raimei anyway. Upset, Raimei lands a punch on Raikou, yelling at him for thinking her to be so weak and taking on the burden alone. Crying, she asks him to come home. But Raikou tells her that it is too late, and makes her take up her sword once more.
Seeing characteristics of their mother in her, he cuts down with his sword, only to find Gau, who in an attempt to intervene, takes Raikou's blow, the one meant for Raimei, saying "I came to accept my punishment." Gau then falls over bleeding, and instead of being angry, he begs for the two to stop fighting, and apologizes for breaking his promise not to tell Raimei as he slips under into comatose. This deeply distresses Raikou, his concern for Gau's condition runs so deep that he contemplates whether Gau will awaken or not, believing that he will not, due to thinking that Gau no longer wanted to see him again. Although Kōga Village's kinjutsushō could have waken Gau up, Raikōu refuses to sacrifice the humans needed to use the kinjutsushō. Later on, Yoite finally wakes Gau up with his own kira. Because of this, Raikou feels indebted to Yoite and even disobeys the leader of the Kairōshū, Hattori from killing Yoite. Eventually, Raikou forfeits his loyalty to the Kairōshū, particularly Hattori, and is denounced as an expendable pawn whom easily replaced. He helps rescue Miharu from the Grey Wolves. Hattori kills Raiko before the activation of the Shinra Banshou. However Raikou manage to survive after the destruction of the Kairoshu.
Raikou is portrayed as rather an easygoing, well-mannered person who takes his responsibilities very seriously. It is implied that he is a sadist, losing control when a target hurt Gau and repeatedly slashed through the corpse until blood spilled everywhere. He keeps his Shirogamon on his right hip, making him having to draw it with his left hand, this may imply that he's left handed. Even though, this doesn't seem to hinder him in battles as he still challenges Raimei with ease, making his left-handed swordmanship an advantage over others. After his second fight with Raimei, he left Shirogamon in the field of spider lillies, replacing it with a tri-sectioned staff, which he uses for 2 to 3 battles in particular. Afterwards, he picks up another, a katana, shown to be used in reverse-grip, in the same style he used to cut Raimei.
- Raikou's and Raimei's past is strongly similar to the Naruto series' Uchiha clan massacre. With Raikou's role as older brother and killer, and Raimei's quest for vengeance, being in exact similarity to Itachi and Sasuke.
- Raikou admitted to not driving ever since he had acquired his driver's license.
- Raikou previously had naturally dark blonde hair color (like his mother and sister), but seemingly dyed it later on to his current salmon pink hair color.
- Raikou was one of the Similarities of Amuro Ray and Flit Asuno from Mobile Suit Gundam Series.
- Raikou, Gau and Raimei shares the Similarities of Amuro Ray, Lalah Sune, and Char Aznable because Raikou accidentally injures Gau to protect Raimei while Amuro in his RX-78-2 Gundam accidentally kills Lalah in the Mobile Armor, Elmeth that prevented Char in the Red Gelgoog from being killed.
- Raikou was based on Harusame because Gau remain loyal to Raikou while Utakata was a Student of Harusame.
- In the anime, Gau admitted that when Raikou is off-duty his personal style is bad.