|Relationships||Mika Shishidō (best friend)|
Tomu Shirasagi (love interest)
Rio Asahina is a former student of Shishidō Academy and the current Junior World Champion of Billiards. She was recruited by her best friend Mika Shishidō for a gamble against Tomu Shirasagi, which she lost and was subsequently kicked out of the school.
Rio Asahina has long brown hair, often kept in a ponytail with a pool ball ponytail holder. She often shows off her huge breasts and body by wearing a short top that shows much of her cleavage and her midriff and short skirts that reveal her long legs.
The cover of Volume 3 feature Asahina wearing a short pink leather halter top with laces on the side, revealing much of her midriff and a pink short-skirt laced up on the side, and wears pink nailpolish.
In her battle with Shirasagi, Asahina wears a bowtie choker with a short sleeveless zippered leather vest, exposing much of her large breasts, a short skirt that reveals her long legs, and high-heeled white boots.
Asahina's return in Macau features her wearing an open-chested leather halter top and a short skirt. When entering the casino, she wears a long qipao-style dress with a breast window and leg slits going to her hips and black heels.
Rio Asahina is from a middle-class family where she managed to enroll in the prestigious Shishido Academy on a scholarship. Her parents owned a popular inn that provided a steady income, but one day, her father gets harrassed by the local Yakuza forcing him to assist in money-laundering. He later consults with a business consultant to help against the Yakuza, but was revealed it was all part of a huge con resulting in all his accounts being emptied out through an illegal bank access. Rio's father died of heart failure from the shock of what happened, then she learned the name of the man who swindled them to be Yumeichiro Shirasagi, Tomu's father. For that, Rio challenges Tomu to get Yumeichiro's current whereabouts.
Being a billiards junior world champion, Rio accels in the sport, and believed beating Tomu would be child's play since he was a newbie and that there could be no possible way he could improve in only ten days of practice. This ultimately became her downfall as she took a bluff of Tomu's to request a do-over of his break shot because of an outside incident, failing to realize that the balls were not moved by accident, but were actually rearranged by Tomu, Mizuhara, and Natsumi while the crowds were distracted. Her scholarship was taken away from her loss by Abidani and was forced to drop out. Rio heads for Taiwan to receive professional billiards training after being inspired by Tomu's drive to win during their match. Before she left, she received a letter from Tomu claiming his father is innocent and that he'd find the one responsible for her family's misfortune. Looking out her plane window, Rio sees Tomu send her off if with balloons modelled after pool table balls. She, too, falls for Tomu.
Rio reunites with Tomu and Mika in Macau City where they had arranged a meeting for Tomu with Shyu Oumei, an old aquaintance of Yumeichiro. Tomu loses a gamble against Oumei involving a pit viper, much to Rio's horror. She managed to help slow down the venom and quickly gets him to the emergency ward to be treated. Tomu gets hospitalized just days before the start of the "Class Reunion". To obtain the necessary funds of a billion yen to enter, Rio, along with Natsumi, Yuka, and Mizuhara play at the casinos using Tomu's acquired money in hopes of winning big. Rio also planned to win extra in order to fund the renovations of her family's hotel. Rio goes to play at the roulette wheel, where she won big, but in the end lost it all, along with Natsumi and Yuka.
Rio is an expert billiards player, where she became the junior world champion. She is able to pull off trick shots such as jumping the cue ball, and at one point, caused it to fly off the table towards Tomu's face.