This character can be seen in the "old type" school swimsuit.

This type of swimwear began to appear in Japanese schools around the 1970s. Its color is usually dark blue, violet or black. There are also the Moe-versions like translucent or white, which is popular among the perverts, since the drenched white swimsuit becomes almost transparent.
Old type school swimsuit has a distinctive front skirt. Unlike the very first version of the school swimsuit (so-called "old old type"), which consists of two separate pieces (panties and "tunic"), in this type of swimwear the back part of the panties is a continuation of the tunic, while the front part is sewn to the inner part of the tunic along two lines, as it shown in the picture. However, if desired, a small area above the pubis can be exposed. There are also might be the tags with the girl's name or class, which can be sewn to the frontside, backside or bottom of the tunic. Old type school swimsuit is mostly worn by girls of the junior or middle schools, although it can be worn by anyone.

More information can be found here and here (Japanese).