Risako Nagisa (凪紗りさこ) is the villainess in the OVA Aika R-16: Virgin Mission (2007, Studio Fantasia). She's voiced by Naoko Suzuki (Japanese) & Tara Platt (English). In Aika Zero she is no longer a villainess.
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.
She is also very powerful. Her combat prowess is probably equal to that of older Aika, as she is only defeated after being attacked from behind by Karen Minamino.