Risako (or Risako-sensei, as she is referred when in school) is introduced as the teacher of Aika Sumeragi, the 16-year old heroine of R-16. In her role as teacher she dresses conservatively and wears her hair up. She is shown to be a serious teacher and strict disciplinarian, harshly punishing Aika for her frequent tardiness.
This seriousness is carried over in her role as secret agent, where she has no problem using physical force against her female students in her attempt to keep Aika and the club from achieving their goal. She is also shown to have a playful and seductive side. For, even when she is still seen by her students as their teacher and club advisor, she lounges around in a bikini and openly teases a young Gusto, shocking her female students. This duplicitousness makes her appear very dangerous, as she is obviously a capable woman in whatever role she plays.
In contrast to the way she dresses in school, Risako seems to prefer sexy and revealing clothing.
Risako has combat skill comparable to a 16-year-old Aika Sumeragi and even gained the upper hand until Karen Minamino's interference. Although Risako was likely stronger than Aika at this point, getting completely overwhelmed and knocked out despite being in a dominant position because of a hot crab likely implies that the gap between the two wasn't massive.
In Aika Zero, Aika seems to have completely surpassed Risako as Miyu was able to instantly take her out with overwhelming speed while Aika seemed to have little trouble with Miyu. This is further shown when Risako was seen on the ground and forced to use a gun for the time in the series against Misumi while Aika was able to take down multiple White Knights with relative ease. Although the battle between Misumi and Risako isn't shown, given her position on the ground and the fact that she sighed after resorting to using firearms in battle for the first time in the series, it's likely that Risako was outmatched in a battle of pure hand-to-hand combat.
The many appearances of Risako Nagisa.
Risako in her role as teacher. Conservatively dressed, she scolds Aika for being late.
Risako is a very strict teacher
As Aika’s teacher, Risako administers harsh punishment!
Students are clearly afraid of Ms Risako
Risako patrols the hallways at school
A more mysterious side to Risako is revealed.
Gusto is obviously distracted by the shapely Risako
A teasing Risako. Outside of school her personality and dress changes.
The girls are not happy with Risako’s behaviour
The skimpy bikini bottom of Risako
Risako relaxes in her revealing bikini
Risako’s “assets”are on display!
Risako makes sure the boys get a good view!
Risako looks seductively at the boys on the trip
Risako is one sexy club advisor!
Sexy Risako knows she drives the boys crazy!
The boys are blown away by Risako!
“Everybody here should just go to hell!”
In the evening, Risako knocks on a door...
...Risako, dressed in revealing lingerie, knocks the student out!
Risako is revealed as a villainess!
Risako is on a secret mission! What is it?
Risako has programmed girls to attack classmates!
Risako is kidnapping a student! She slings her over her shoulder!
Risako and her forces have incapacitated most of the students!
Risako is kidnapping Karen.
A humiliating position for Karen on Risako’s shoulder!
Risako guarded by one of the girls she programmed to obey her!
Still holding her kidnap victim, Risako realizes it won’t be an easy escape!
Someone is here to stop Risako!
A sniper is serious about stopping Risako!
Risako is caught in a searchlight!
Risako tosses a gun to her guard!
The guard is taken out by the sniper!
Risako has almost escaped with Karen!
Aika charges toward Risako!
Risako hears someone coming...
...she turns around and...
...and gets speared by Aika!
Risako recovers after being speared!
Risako escapes as Aika watches
Risako on board a helicopter
Risako reports her kidnap attempt was unsuccessful
Risako will need to make another attempt
This bikini is even skimpier than the white one!
Risako approaches the beach in another revealing swimsuit!
Ready to make another kidnapping attempt
Risako arrives with her henchwomen
Aika sees Risako approaching
All of Risako’s henchwomen have been defeated!
Risako is ready to use her assets in battle!
Gusto is intimidated by the boobs of Risako!
Gusto tries to stop Risako’s evil plans
Risako in obvious control of her own sexuality.
Gusto has just been tossed aside by Risako’s cleavage!
Risako tells Karen to come with her!
Aika leaps to Karen’s defense!
The arrogant Risako is ready to destroy Aika!
Risako has no trouble beating down one of her students.
Risako doesn’t look worries as Aika tries to punch her!
Risako easily avoids Aika’s punches!
It is so easy for Risako!
Risako effortlessly dodges Aika’s kick!
Risako takes Aika down with a leg sweep
Powerful Risako looks down on her student Aika!
The villainess puts her foot on Aika’s chest!
Risako tells Aika she didn’t wash enough windows!
There were too many windows!
Risako at her most dominant.
That is why it was a perfect punishment!
Aika twists Risako’s ankle...
Risako’s reaction to Aika twisting her ankle
Risako lands a huge punch!
Risako giving Aika a brutal kick to her Stomach
Risako regains the upper hand!
Risako in complete control!
The evil Risako throws her student to the ground!
Aika has a painful landing!
The scantily clad Risako moves in to finish her opponent off!
Risako jumps on top and pins Aika down!
Risako Choking Aika with brutal strength
”Sweet dreams,” Risako says as she is ready to knock out Aika!
Suddenly Risako feels something on her back!
Someone is attacking Risako from behind!
Risako turns to see who is behind her
It’s Karen! Risako sends her flying with a violent punch!
Aika sees Karen get hit by Risako...
Aika throws sand in Risako’s face
Risako is temporarily blinded by the sand!
Aika headbuts Risako in the face!
Risako is reeling from Aika’s blow!
Aika gains the advantage!
Risako is rocked by a big kick from Aika!
Aika follows up with another kick to Risako!
Risako desperately tries to hold her ground!
Aika’s final kick knocks Risako out!
After getting knocked out, Risako is tied up!
Gusto binds Risako’s wrists
Risako says she already gave up and asks if it is really necessary to tie her hands.
Risako is still defiant after being tied up!
Aika is angry as Risako refuses to cooperate!
”They’re listening,” Risako tells Aika to give up!
“What is it now Karen?” Risako demands.
Karen shoots Risako with a tranq gun!
Risako losing consciousness!
Risako about to be stripped by Karen!
Risako is out cold as Karen searches her bathing suit!
Karen holds the transmitter she discovered in Risako’s bikini
Gusto is shocked to see that Aika and Karen have defeated Risako!
After having the transmitter removed from her bikini by Karen, Risako‘s last hope for help is gone!
Gusto slings a defeated and unconscious Risako over his shoulder!
A humiliating position for the arrogant Risako!
Risako, still knocked out, being carried by Gusto.
No longer intimidated by Risako, Gusto complains about having to haul her away!
Bottoms up! Risako and her henchwomen, captured and unconscious, will be tied up and put in a storeroom!
After defeating and capturing Risako, they are off for more adventure!
After Risako has been defeated by her students, she gets draped over Gusto’s shoulder.