Riko despises Vlad for what he had done to her in the past. Aside from treating her as a means to create a "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 seething with vengance
Riko knows of his special condition, and thus loves teasing him because of it. She isn't 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.
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 dwarf 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