The Sealing Key is a key in a shape of a sea horse given to Taro by Sara to allow him to enter Gaito's castle. Aqua Regina used it to seal the Panthalassa Clan and gave it to Hippo to guard it but the key was stolen by Sara and she removed the seal.
In the Pure season, the key doesn't really appear, though, in the manga, it is used to cut Mikeru's whips when he attacks the secondary trio (Coco, Noel, and Karen). Sara used the key to abduct Tarou to Gaito's castle. He can breathe underwater with the help of the key.
The key is golden and shaped like a seahorse. Around the head of the seahorse is a golden disk with small decorations on it.
The sealing key can be used to seal and unseal the Panthalassa Kingdom. Hippo has the power to turn it into a staff, which is mainly used to break stuff like glass tanks or doors. Hippo can turn into the Hippocampus with the key and in his human form he can transform the key into a staff. The key can lead a person to the Panthalassa Kingdom and enables the user to breathe under water. In the manga the key can be used to cut Mikeru's whips.