The Floodgate Keeper is a character from The Legend of Zelda: Oracle of Seasons. He is in charge of operating the floodgate in Spool Swamp. If Link enters the Floodgate Keeper's house and flips a switch he will receive the Floodgate Key. Also, a pool of water outside the house will dry up, revealing a secret passage. This underground passage is used to find the floodgate lock, where Link can use the Floodgate Key. This chain of events is necessary in order to gain entry to the Poison Moth's Lair, the third dungeon in the game.
Interestingly, when Link talks to the Floodgate Keeper at later points in the game, he will repeat the same dialogue Impa spoke when Link first met her.