Reiko was born into a rich family without any love or interest for her. Consequently, she studied very hard to get the acceptance of her family, neglecting her social life in turn. Unable to get this acceptance, she developed hatred for her family and for the world itself. Seeing this opportunity, Rogles used her to fuse with the dark Valis sword that he created through her negative emotions so that she could become the dark Valis warrior and fight for him.
Yuuko found out about Reiko's transformation during her first adventure and tried to make her recover her conscience, but Reiko, blinded by rage, fought against Yuuko only to be mortally wounded and agonize in Yuuko's arms. Here, she tells Yuuko the reasons why she became her opponent only to receive in answer Yuuko's feelings of friendship and concern for her. Knowing that she had a friend in the world, she died peacefully in her best friend's arms.
In spite of her death, Reiko continued to help Yuuko from the afterlife, warning her on several occasions in dreams and apparitions about different threats Vanity and the real world.
In the beginning, Reiko was a lonely girl that talked only to few people such as Yuuko. She was hatred about everything in her life and wanted to destroy everithing in revenge for her destiny. Until she fought Yuuko and became friends with her before diying.
In her afterlife, Reiko's attitude changes to a possitive and reflexive one and is always willing to help Yuuko and make her enter in reason when she seems confused and/or scared.
Reiko has a great intelligence, product of her hard studies.
As the dark Valis warrior, she uses many powers similar to Yuuko's ones. She can jump highly, run, slide and use her sword to attack with energy waves. She can also cast many powerful spells like a spiral wave chaser and a deadly flash.
In her afterlife (as a spirit) she seems to have the hability to appear in dreams (as she did in Valis II from PC-88/98 and SX68) and even materialize temporally when in Vanity (as she did in all Valis III versions) thus being logical because Vanity is in the dreamworld.
Reiko appears in every existing Valis game and compilation, just as Yuuko has, with the single exception of Super Valis IV (SNES).
In the main series, she appears during the first game as Yuuko's evil counterpart. Afterward, she appears during Valis II to help Yuuko from the afterlife, and again in Valis III questioning Yuuko's motivations.
In Valis SD she appears to be more kind and even to act in a cute manner proper of the parody.
In Valis IV Reiko makes a cameo in Yuuko's flashback when she remembers Reiko dying in her arms.
In both Mugen Senshi Valis mangas (old and new) she retains her role from the first game. She is portrayed comically as a ghost during the end of the first manga.
In the new manga she seems to dislike most of Rogles' henchmen although working for him, Voldes being the only one to whom she can relate. She seems to not like dirty tricks, as she killed Izard herself when discovering he abducted (and afterward hypnotized) Yuuko when she was fighting against the Valis warrior.
In the Valis SD short mangas, she acts as in the game version, although she is the one who appears in front of Yuuko to warn her about Megas instead of Yuuko confronting Megas himself. In turn, Yuuko gets angry because of being interrupted when she was eating ice cream.
- Although being dead, Reiko seems to have a great role along many games of the series.
- Reiko's name derivates from the kanjis rei- (麗) meaning "lovely", and ko (子) meaning "girl" or "child", thus her name means "lovely girl". This meaning makes sense since her being considered as a "best friend" by Yuuko.
- Reiko has many opposite facts to Yuuko in acord of being her counterpart:
- While Yuuko's hair is long and blue, reiko's one is red and short.
- While Yuuko is shy and somehow ingenuous, Reiko is determined and intelligent.
- While Yuuko's armor is golden, Reiko's one is black.
- While Yuuko lived with a mid-class affective family, Reiko lived with a high class but cold family.
- Because of those facts she is considered as Yuuko's rival in the series.
- Reiko's hair is colored pink in the Valis II's PC-88/98 and SX68 versions, this could be due to graphical limitations.
- In that same game, Yuuko can unlock Reiko's armor and use It.
- Reiko's surname is sometimes mentioned as Kisaragi instead of Kirishima.
- If the player uses Valna or Cham in the Valis III stage where Reiko appears, they will not recognize her, being this the only time they meet in the main series.
- Cham from the afformentioned game bears a slight resemblance to Reiko.
- Reiko's full name seems to be the inspiration for the name of the protagonist of another game adapted by Telenet, Reika Kirishima from Time Gal.
- Reiko's underwear can be seen when jumping just like Yuuko's one in the PCE-CD version of Valis I.
- Strangely in fanworks, she is frequently depicted wearing black underwear instead of white (the same color of Yuuko's) as she wears in the game.