TV ANIME: Onsen Yousei Hakone-chan [BGM: ???]

ORIGINAL TITLE: 温泉幼精ハコネちゃん

JAPANESE WIKIPEDIA LINK

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SYNOPSIS

Hakone is a hot spring spirit who descended to a spa resort on Earth. However, with her long slumber, she looks like a little girl on the outside.The high school boy Tōya stumbles across her at a hot spring and ends up joining Hakone in retrieving her original powers.

(Source: ANN)