After her parents died in a car crash when she was 8, Vlad Dracula took Riko in by pretending to be her relative but locked her up in an underground dungeon to conduct an experiment in creating an ideal Lupin V. It turned out that Riko didn't inherit the Lupin bloodline's superior gene, she was branded a failure. She had been imprisoned there for a long time and was forced to eat rotten meat and drink muddy water to survive. It was said that because of her malnutrition, she has a present small stature and because she could only wear ragged clothes in the whole time of her imprisonment, she developed a love for fancy clothes.
Riko reveals that she is the Butei Killer. Although she states that the name is wrong as she never killed any of her victims, but had instead kidnapped those who had disappeared. She appeared to have an inferiority complex concerning the first Lupin and wanted to escape his shadow. Her plan was to make Aria and Kinji become partners so it could mirror the situation of Sherlock Holmes and Watson. Defeating both of them would mean she would become better than her ancestor Arsène Lupin who couldn't defeat Sherlock 100 years ago. She despises being called "4th" because she feels it removes her personal identity, as such she prefers "Riko".
Riko and Aria have similar traits: small stature, the title of "Quadra", twin-tail hairsyle (in Riko's case, her hair is not entirely split in half unlike Aria), and being the fourth generation of their families. She has a height of 4'10" and weighs 96 pounds. the only different of them is her hair style and bust size.
Her most valued possession is a cross made of Irokane. This cross allows Riko to control her hair at will. She hides it under her tongue when she was imprisoned naked in an underground dungeon. Vlad eventually steals this cross, prompting a series of events in which Kinji and Aria retrieve it by infiltrating Vlad's place.
She usually puts on a facade of stupidity, but some times, usually during tough situations, she breaks the facade. Kinji calls this side of her personality "Dark Riko". With the facade removed she uses rough language, her eyes emit an intimidating glare and she refers to Aria as Holmes (French: Olmes) instead of Aria. She is also one of the few people who knows Kinji's secret.
Riko loves to sexually tease Kinji. However, after Vlad is captured she seems to be developing true feelings of love for him as she can no longer do aggresive things to him (which, according to Shirayuki's book, states that a girl can't do aggresive things to the boy she really likes,) but is starting to act shy and nervous at times. This is due to him pickpocketing her beloved cross from Vlad's possession during the roof battle and returning it to her despite her earlier betrayal. Also, by Kinji informing her that by defeating Vlad she has achieved her goal of surpassing Lupin the First, who had barely been able to fight Vlad to a draw with the D'Arc twins.
- Riko easily gets aroused.
- All her clothes can double as parachutes.
- Riko's height is directly caused by the lack of nutrients from her childhood imprisonment.
- Riko's surname is a tribute to Fujiko Mine from Monkey Punch's Lupin III.
- The reason why Riko isn't called the "4th" is her tetraphobia.
While she is a descendant of Arsene Lupin, Hidan no Aria is in no way connected to the Lupin III franchise