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|Main Ability||Gigantic Pie Pile Piper|
|Manga Debut||Chapter 115|
As described by her comrades, Rio cares deeply about her juniors, and is willing to guide and help them in any way. Rio used to be an ambitious person. However, due to her injury, Rio becomes pessimistic. Thus, she believes that dreams can't be attained so easily, referring to them as "boring ideals". Rio will laugh over someone who aims higher than their capabilities. For instance, when she insults Nozomi, who wants to defeat the Five Butts. Despite this, Rio shows sportsmanship. She congratulates Nozomi over her victory and is satisfied with their race. Rio describes her dream is just to bring some "excitement" to the Funabashi Keijo Stadium.
Rio once dreamed of defeating the Five Butts and bringing the excitement to the small venue Funabashi. At some point, however, she had a race with Meiko Ginya from the Chiba Branch. Meiko viewed Rio as a promising player. After being defeated by Meiko, Rio received a scar on her face, and was heavily injured. Thus, she needed to hiatus from participating in Keijo for the sake of her recovery. While crying, Rio thought it was the end of her career.
The night before the preliminaries of the Gulf Coast Cup, Rio appeared inside the Funabashi Keijo Stadium. She told Yoshinaga to take a bath immediately. Confused, Nozomi Kaminashi asked to Yoshinaga who she was. In turn, Yoshinaga explained about Rio. After the preliminaries, Rio was taking a bath, accompanied by Aota and Kigawa who washed her butt. Before leaving, she warned Nozomi about how to survive. The next day, Rio and Kigawa oversaw Nozomi when the Hyougo Branch player defeated Doi. The next day, Rio and her comrades, consisting of Aota, Kigawa and Mitsuko Hoshi were advancing to the finals. Rio was surprised that Nozomi managed to reach the finals. As the match started, Nozomi assaulted Rio with her "Meteor Hip". However, Nozomi was unable to push her at all, realizing how strong Rio was. Spininig her boobs, Rio decreased the temperature of the "Hot Smoker" Land type, creating a huge amount of fogs to trap Nozomi.
After Nozomi was assaulted by Aota, Kigawa and Mitsuko, Rio praised Nozomi since she was still able to hold off. When Nozomi declared that she wanted to take down the Five Butts, Rio laughed out loud before landing a fatal blow onto Nozomi's backhead. Along with Aota, Rio tried to finish Nozomi off. However, both of them were ambushed by Kazane Aoba. Rio questioned if Kazane wanted to talk about the dreams that couldn’t be attained so easily. In turn, Kazane stated that nothing could change if they didn’t take a try. Realizing that it’s pointless to talk with both Kazane and Nozomi, Rio was pissed off and blew Kazane away. After Aota and Kigawa prevented Nozomi to move, Rio assaulted her with the "Water Moon Killer". Despite this, Nozomi was still able to survive. Shortly after, Kazane returned to rescue Nozomi, and both of them were able to defeat Aota. 
After Mitsuko was defeated by Nozomi, Rio recalled her past when she was hospitalized. Ripping off her bikini tops, Rio revealed her K-cup boobs and overwhelmed Nozomi by using them. When she was preoccupied by Nozomi, Kazane prepared an ancient technique she obtained by scanning a pharaoh's butt.  When Kazane used the "Neith Butt Bolg", Rio was surprised since Kazane was perfectly able to dodge her attacks. Kazane revealed that her technique wouldn't be avoided as it always targeted the opponent’s crotch. When Kazane was about to finish Rio off, Kigawa stepped between them and protected Rio. After being defeated, Kigawa asked Rio to defeat both Kazane and Nozomi. Rio then lured Kazane to attack her crotch before countering Kazane with her gigantic boobs, defeating Kazane. This made Nozomi realized that Rio was risking her life to win.
Nozomi rushed towards Rio and the two engaged in a fierce fight. Due to this, the staff and spectators were amazed. Afterwards, Rio decided to use the "Gigantic Pie Pile Piper" technique to use her boobs as a drill in order to stab Nozomi. On the other hand, Nozomi used her specialty, the "Vacuum Butt Cannon". Rio ultimately realized that she has exhausted and was unable to block the superior player. Nozomi pushed Rio away and made her sunk, winning the competition. After being defeated, Rio apologized to her comrades. However, all of them were proud of her, before suggesting Rio to see at the spectators. Rio noticed that the spectators were satisfied with her race. She then recalled that she only wanted to bring some excitement to the Funabashi Keijo Stadium. While crying, Rio thanked them.
As shown in the flashback, after their race, Rio approached both Nozomi and Kazane. Rio congratulated Nozomi before inviting her to come back to Funabashi someday, claiming that she would get stronger. Thus, the two would be able to clash their stronger butts again.
Butt Graduation Festival Arc
Rio is the most veteran player of the Funabashi Branch, and is respected by a lot of Funabashi Branch's players. With her large boobs, Rio specializes in using boob techniques.
Enhanced Durability: Rio is able to withstand a great physical damage. It's worth noting that she survives "Meteor Hip" a couple times, albeit Nozomi targets her backhead.
By utilizing her breasts which are extremely big and long, Rio can use various chest techniques, engaged in a fierce fight with Nozomi Kaminashi. Below is a list of techniques she used during the clash with Nozomi:
- K-cup Magnum (Kカップマグナム, K kappu Magunamu)
- Massive Boob Bomb (巨乳砲弾, Kyonyū Hōdan)
- X Breast Assault (X乳斬波, X Chichi Zanba)
- Explosive Breast Fist (爆烈乳拳, Bakuretsu Nyūken)
- Exploding Boob Sniper (爆乳スナイパー, Bakunyū Sunaipā)
- Vector Breast Cannon (ベクター乳キャノン, Bekutā Chichi Kyanon)
- Boob's Style Final Heaven (乳式ファイナルヘヴン, Chichi Shiki Fainaru Hebun)
- Gigantic Pie Pile Piper (ギガンティック・パイ・パイル・パイパー, Gigantikku Pai Pairu Paipā): This is the advanced version of "Pie Pile Piper". Rio, by using her large and long breasts, twirling them up, which later takes the form of a drill. She then uses them to stab the opponents.
Breast Copter (乳コプター, Chichi Koputā): Rio horizontally swings her breasts, mimicking how a helicopter flight. She then lands her her breasts onto her opponent's head. Delivering a fatal blow.
Full-Power Chest Bat (全力乳バット , Zenryoku Chichi Batto): Rio swings her breasts rightward with a great amount of force to strike the opponent. Delivering a fatal blow that it can cause her opponent choked.
Breast Stick (乳意棒, Nyūi Bō): Rio swings her breasts upward, targeting her opponent's jaw.
Water Moon Killer (水月殺し, Suigetsu Goroshi): A combination technique she performed alongside Aota and Kigawa. Started by locking their opponent's movement, Aota and Kigawa hold each of their opponent's boobs. Aota and Kigawa then expose their chest, while Rio assaults the opponent with a great amount of force. Delivering a fatal blow to the solar plexus.
- (To Nozomi Kaminashi) "Icarus aimed too high for the sky before he lost his wings. He fell to the earth and destroyed his own body. Then who will you be? A butt that embraces its ideals and falls... or a butt that blooms on the surface like a flower? In order to survive in this world... you must have the right butt."
Races & Events
This includes official races, unofficial races, and trial matches:
- Rio Kosugi, Aota, Mitsuko Hoshi and Kigawa vs. Nozomi Kaminashi (Interrupted)
- Rio Kosugi vs. Kazane Aoba (Win)
- Rio Kosugi vs. Nozomi Kaminashi (Loss)
- Rio Kosugi vs. Saya Kogatana (Loss)
- Her surname, (小杉, Kosugi) literally means "small cedar".