Professor
Dr. Mizumi is a scientist referred as the crazy old man by ... as he was not named in the games.
In Ocarina of Time he is a chemistry scientist, and makes medicines at the Marine Research Laboratory.He also loves the taste of eye ball frogs.
At Majora's Mask he works at the Lake Hylia Laboratory as a marine scientist. He worries about the pollution in the sea, and takes care for Lulu's eggs until they hatch.
Dr. Mizumi was of an unknown race. Link encountered a nearly drowned Rouru in a lake, and decided to bring him to Dr. Mizumi. Mizumi offered to let Rouru stay at his laboratory until he recovered. Soon after, though, Rouru had a fit and destoyed most of the doctor's experiments and equipment. After Rouru was healed, he left with the rest of his flock and Link offered to help rebuild Mizumi's life work. [1]
Dr. Mizumi's Terminan counterpart appeared in the Majora's Mask Manga as a fisherman. He mistakes Link for the Zora, Mikau, since he is wearing the respective mask. Link gives him Lulu's egg for safe keeping and study.[2]
Trivia
Dr. Mizumi's name is spelled with two "U'"s (Mizuumi) in the Majora's Mask Manga.[3]