Kario

Kario  is the het ship between Karma Akabane and Rio Nakamura from the Assassination Classroom fandom.

Canon
Rio and Karma often agreed on decisions because of their similar mindsets, it's said(by the students) that they could finish each other's thoughts. When Nagisa proposed to save Korosensei, they were the two the most against it, since they believed that their bond with their sensei is assassin-target, and that they cherished that relationship/thought. Rio acted as Karma's second-in-command in the battle.

When Nagisa kisses Kaede, the first thing Karma and Rio do is take out their phones and take pictures, which they use to tease Nagisa later. On Valentine's Day, they decided to tease Kaede with the picture they took of her and Nagisa kissing. When they realize that Kaede had genuine romantic feelings for Nagisa, they smiled evilly and said if they could get them together they can "tease two lovebirds with one stone."

Later, the two spy on Kaede when she gives chocolates to Nagisa, Rio indirectly confesses to Karma that she wouldn't act on her own feelings for Nagisa, for Kaede's sake.

Fanon
They are a decently popular pairing in the fandom, most ship them for their compatibility and similarities.

Both Karma and Rio had feelings for Nagisa, but when Kaede got him they turned to each other and developed feelings. They are partners in crime and do everything together. Many like to explore how their day spent spying on couples around the school ended, since they didn't have a valentine.

Fandom
FAN FICTION 

TUMBLR 

Trivia

 * Rio's favorite food is pizza and she hates boring men.
 * Karma's favorite drinks are from the "Simmered au lait" series.
 * Karma and Rio are both prodigies and rank the highest of Class E.