Kokona Tagawa

Kokona is Megu Kataoka's former classmate. She essentially blackmails Megu to do help her study after Kokona nearly drowned from failing to learn how to swim.

Personality
Even though Kokona is a popular person, (as she never seems to be alone in the manga) she is very selfish and doesn't think about anyone else except herself. She seems light headed since she went swimming in the sea after only practicing once with Megu. Because she isn't good at studying by herself, she blackmails Megu to help her study.

Plot
Megu tought Kokona how to swim since Kokona wanted to go to the sea with some friends and an unknown crush and she didn't want to embarass herself in front of them. Because she always made excuses for not coming to the training, she nearly drowned in the sea due to her lack of experience in swimming. Kokona blamed Megu for that and blackmailed her to help her study. After her "dream" with Megu, Megu finally could free herself from Kokona's blackmailing after she witnessed Kokona swimming in school.

Megu Kataoka
Megu and Kokona were friends since middle school, Kokona always hanged out with Megu and one day asks her if she could teach her how to swim to impress a guy in the beach. She failed at learning how to swim because of her selfishness and starts using Megu just as a tool. Korosensei, Nagisa and Kayano helps Megu in her situation of Kokona in her 'dream'.

Trivia

 * Kokona Tagawa omitted in the anime.
 * Her last appearance is in Chapter 118, eating together with Kaho Tsuchiya at Class E's restaurant.
 * She had a traumatic fear of water, swimming and the sea.