Professor

Dr. Mizumi is a strange doctor and chemist that appears in Ocarina of Time and Majora's Mask; in both games the doctor has a minor sidequest-related role.

Ocarina of Time
Dr. Mizumi is a scientist in Ocarina of Time, who lives in Lakeside Laboratory located above Lake Hylia. He is a chemistry scientist, putting his knowledge into medicinal practice at his laboratory. When Link first meets the doctor, he mentions a mutual friendship with Granny, who is the potion shop old lady of Kakariko Village.

The pool in his laboratory is eight meters deep; he will watch Link dive and make remarks based on the depths reaches (and each remark corresponds to the depth Link can reach with any given scale). If Link can touch the bottom of the pool, he will give him a Piece of Heart and tell him he is the best diver in the world. Link needs a Golden Scale to do this, however.

Mizumi is a part of the Biggoron's 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 excitedly thinks the frog is for making fried eyeballs, which he finds delicious, but Link corrects him and explains he brough the frog because of its medicinal properties. Mizumi creates a solution of eyedrops for Biggoron from the Eyeball Frog.

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 by keeping them in a special tank until they hatch. Dr. Mizumi says that if the Zora Eggs aren't kept in the specifically-made aquarium they may die, due to the abnormal weather patterns going on that are causing a rise in the ocean's temperature. Dr. Mizumi also is one of the few characters to reference the seahorse, even going so far as to offer Link a hint as to where the seahorse is.

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 the life work of Mizumi.

Dr. Mizumi's Terminan manga 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.