Kid

A promised past and returning where one belongs. The underwater theme park, LeMU. Takeshi Kuranari, one of the park guests, is suddenly involved in a freak accident. And he finds himself stuck in LeMU with a number of other survivors. Because of the system's failure, the water pressure will slowly crush the facility. The time limit till implosion is 170 hours and 17 minutes. Will they be able to escape to the surface!? This webcomic is based on the visual novel Ever17 ~ the out of infinity. Wikipedia

This series follow the adventure of a middle school kid named Shitara You. In order to avenge his father, he has to master several techniques that will make him invincible.