Professor

Ocarina of Time
Dr. Mizumi is a scientist in Ocarina of Time, who lives in Lakeside Laboratory, located near Lake Hylia. He is a chemistry scientist, and makes medicines at his laboratory.

Mizumi is a part of the Biggoron Sword trading sequence. After an Eyeball Frog is given to Link by King Zora, he must rush down to the laboratory and give it to the doctor. At first, the doctor thinks the frog is for him to eat and is ecstatic, but then realizes that it is actually for Biggoron. Mizumi makes the frog into eyedrops for Biggoron.

Majora's Mask
In Majora's Mask, Dr. Mizumi works at the Marine Research Laboratory as a marine scientist at Great Bay. He takes care of Lulu's eggs until they hatch by keeping them in a special tank. If the Zora Eggs aren't kept in the specifically-made aquarium, they may die, due to the abnormal weather patterns going on.

Manga
In the Ocarina of Time Manga, Dr. Mizumi is introduced after Link encounters a nearly drowned Rouru in a lake, and decides to bring the Watarara to Dr. Mizumi. Mizumi offers to let Rouru stay at his laboratory until he recovers. Soon after, though, Rouru has a fit and destroys most of the doctor's experiments and equipment. After Rouru is healed, he leaves with the rest of his flock and Link offers to help rebuild Mizumi's life work.

Dr. Mizumi's Terminan counterpart appears 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.

Trivia

 * Dr. Mizumi's name is spelled with two "U'"s (Mizuumi) in the Majora's Mask Manga.
 * The Skull Mask is very similar in appearance to Dr. Mizumi, and may be loosely based off of his skeletal structure.