It's Lugia, one of the Tower duo with Ho-Oh, even sometimes as Ho-Oh's polar opposite. It's possesses the ability to control the weather; most notably, it can calm and give rise to storms, so it has the ability to overwhelm and calm the Legendary birds of Kanto down and becomes the trio master of them. It's highly intelligent, and because of the devastating power it could accidentally inflict, it isolates itself deep underwater and tends to sleep in solitude.