Kinji Tohyama[]
Riko Kissing Kinji
Riko knows of his special condition and loves teasing him because of it. She is not afraid of making a move on him and attempting to seduce him, much to the dismay of the other girls after Kinji's love. However as the series progressed, Riko began putting more feelings into her advances and it is implied she is in genuine love with him rather than lusting for him. Also, she tends to make tight situations with Aria worst by "adding more fuel to the fire" causing unnecessary stress for him.
Similar to Aria and Shirayuki, Kinji has accidentally proposed to Riko without realizing it when celebrating her birthday. Kinji mixed up the birthday present he bought her with his newly made stamp and accidentally handed Riko a very feminine stamp marked with Tohyama and asked her to follow him. Riko was stunned then thrilled and praised him for his originality, mistaking this as a marriage proposal. This is the third time Kinji had accidentally proposed to a girl on their birthday without knowing, the first being to Aria with a ring, the second time was to Shirayuki with roses.
Aria H. Kanzaki[]
Initially, Riko appeared to be quite fond of Aria because she could tease her and Kinji to no end. However when Riko was revealed to be the Butei Killer, she unveiled her morbid obsession and hatred for Aria. Since she was a Lupin and Aria was a Holmes, Riko went to great lengths to ensure the duel they would have would rival if not surpass the duel their ancestors had in the past. Following her defeat and banishment from IU, she began to put more faith into Aria and stopped trying to defeat her in order to surpass her great-grandfather. Nowadays, they are on good terms although now they are friends and rivals for Kinji's love.
She also understands that Aria is Kinji's best trigger for Hysteria Mode, so she isn't afraid to get intimate with Aria in order to trigger Kinji
Sayonaki Tooru (Vlad)[]
Vlad Holding Riko
Riko despises Vlad for what he had done to her in the past. Aside from treating her as a means to create the "perfect" Lupin, he had subjected her to inhumane treatment that stunted her physical growth and stolen her mother's Rosary from her. The mere mention of his name can instantly ruin her mood and make her seethe with vengeance.