『Riko Mine』

A female student at Butei High School and a friend of Kinji's. She is enrolled in the Inquesta department along with Kinji. Although often considered the biggest idiot in Kinji's class, this is a deliberate burikko facade which she drops when she is forced into dangerous circumstances. She has a talent for intelligence assessment and, at Butei High, gossip. She enjoys customizing her uniform in an Elegant Gothic Lolita style. She wields two Walther P99s and two knives and, much like Aria, has earned the title of"Quadra".

Near the end of the first volume, Riko reveals that she is a descendant of Arsène Lupin. Her real name is Riko Mine Lupin IV. Riko also reveals that she is the Butei Killer. She appears to have an inferiority complex towards the first Lupin and wants to exceed his legacy. The reason for this, and why she despises being referred to by her generation's number, is due to the circumstances under which she grew up. Orphaned at a young age, she was taken in by Vlad/Toru but kept locked in a cell with very little food and muddy water, and eventually was used as an experimental subject by Vlad/Toru. During this time he repeatedly told her that she's worthless(since she hadn't inherited any of the past Lupins' abilities) and that her only redeeming quality was that she could be used to produce another generation of the Lupin name, Lupin the V. Riko was eventually released from her captivity by Vlad/Toru and promised complete freedom provided that she was able to surpass the original Lupin. Her original plan was to recreate the historical encounter between the original Holmes and Lupin, with Aria and Kinji playing the roles of Sherlock and Watson. By defeating both of them she would surpass her ancestor since the original encounter ended in a draw. This plan fails, but Riko's able to convince them to aid her in recovering a memento from Vlad/Toru.

After helping Kinji and Aria defeat Vlad, Riko helps Aria by providing evidence that Vlad/Toru, and not her mother, Kanae Kanzaki, was truly behind all of the Butei Killer murders, and so reducing Kanae's sentence and delaying her impending execution. Riko appears to have feelings for Kinji, but these feelings may not be completely healthy because she constantly attempts to seduce Kinji- attempts which fail since Kinji is able to see through her machinations. She kisses Kinji after he points out that by defeating Vlad/Toru, she has surpassed her ancestor and can finally take pride in herself, perhaps sparking a more healthy interest in him. Riko and Aria have several characteristics in common: they are short, both have earned Quadra status, both wear their hair in twin-tails, both are the third generation scions of famous ancestors, and lastly, both are romantically interested in Kinji.