The Drunken Coach is introduced as the coach on top of a mountain on the U-17 grounds that coach Saitō had the middle school losers climb up,
The 27 eliminated middle schoolers climb a mountain only to find the eliminated high schoolers from the start of the series. The Coach instructs the middle schoolers to perform weird tasks that include: digging a large hole for him to urinate in, sleeping in a cave, training during midnight while it's raining, jogging through the forest against the high schoolers while chased by eagles, and instructing Ryoma, Kei, and Kenya to steal sake from the main camp... while running through lasers.

His strange tasks are not in vain though - the eliminated middle schoolers realized their improved techniques (Ryoma and Kintarō can hit 5 balls, Kenya has better balance and improved speed, Sakata can run in zig zags, Gakuto can do a low moonsault, Sanada has a larger and stronger black aura, Kei lost weight, etc.)

Two years prior to the storyline, it is unknown when, but during this time, Byōdōin Hōō went to train in the Mountains with the Drunken Coach.
One year prior to the storyline, it is unknown when, but during this time, Tokugawa Kazuya went to train in the Mountains with the Drunken Coach.

he night prior to the Genius 10 Challenge, Coach Nyudō sends the jerseys he had taken from the middle schoolers back to them. The jerseys were neatly folded within a large box and then dropped off at the official U-17 Training Camp by airplane. The box would then land in one of the tennis courts before Oni, who then returns the jerseys to the middle schoolers. It is unknown if he was the person who piloted the airplane.
After the G10 matches were completed, he was revealed to be the head coach of the U-17 camp.

(Source: Prince of Tennis Wikia)