Little sister: Baka Cirno

Cirno

Nine plus nine equals ninety-nine! I'm the strongest math teacher!

Cirno

Any questions from my perfectly accurate lesson?

Yukiko Amagi

Did you hear what Adachi-san said? About Yu-kun collecting little sisters?

Yukiko Amagi

(Why would he need an army of little sisters...?)

Chie Satonaka

First Nanako-chan, then every girl he helps starts calling him 'big bro'... This is getting weird!

Yosuke Hanamura

Partner, what are you doing...

Yu Narukami

You. You're perfect for my collection.

Cirno

Of course I am! I'm the strongest at everything!

Yu Narukami

Cirno, will you be my little sister?

Cirno

Yes! Now I'll be the strongest little sister!

Cirno

Does this mean I can freeze all your other little sisters?