Onsenman

(from mangapedia):

This is a story of the struggles of Onsenman who tries his best to relax the mentally and physically exhausted people of today by taking a hot spring bath. The author, Kazuhiko Shimamoto, says, "Sometimes I want to make something funny and silly, something that is not hot-blooded" but the story gradually becomes more and more passionate and grand in scale, far exceeding the original concept.

A miracle happened to one of the hot spring eggs that was being made in Owakudani in Hakone, and the legendary hero Onsenman was born from inside. Immediately after his birth, Onsenman flew to Tokyo and suggested to Nami Katsuta, a student who had been skipping school to play at an arcade, to relieve her fatigue in a hot spring. Nami rejects Onsenman's suggestion, but Onsenman forces her to enter a hot spring.

At first, Nami is negative about hot springs, but she soon realizes their charm and becomes a fan. After witnessing this, the Onsenman flies away to heal the modern man who, like Nami, is also exhausted.

The first volume includes a behind the scenes bonus manga about the making of the story, and the third volume includes a secret story about the research trip. The story was serialized in "Monthly Shonen Ace" from the April 1995 issue to the February 1997 issue.