Guardian's Awakening. After Atlantis, #1

Tane is the beta in his team of superheroes--the muscle who lets his team do the thinking. He and his friends defend their small town on the Atlantean Isles from the experimental robots of a neighboring mad scientist, earning enough bounty to live on. When Tane discovers a mysterious gem that threatens to take over his mind, he accidentally drags his friends into becoming the crew of the mysterious Mercury Island.
In addition, the island accepts Tane as its Guardian, making him the leader of the group. This sparks a cascade of conflicts between himself and the previous leader, Sebastian, who doubts and questions Tane at every turn. To make matters worse, they discover that a girl lies in stasis deep within the island--and she is the most powerful super of all. Now Tane is in deeper and deeper trouble as the girl's powers awaken, attracting the attention of supervillains and monsters alike.
But she is the key to making Mercury Island fly again. Tane has only begun to grow into his Guardian role, but his enemies--and allies--may kill him first.