During the conversation between Ryūji and Masatsugu Keikain about the seal and this Shikigami. It seems that this Shikigami is the most powerful shikigami Ryūji is using, knowing that he must put all his strength for summoning it.
The Suiryū rises from water or from the moat's water. By using this shikigami, the user must not lie about his movements. It can release seals that contains water, revealing how the seal has been created or placed.