Okay, I want to clarify something here. In the English dub, this man is called "Master Roshi." In Japan, he is referred to as "Muten Roshi", which translates to "Turtle Elder." His actual name is Kamesennin (which is why his house is named "Kame House," and his energy-wave technique is "Kamehameha").

Now that's clear, let me introduce you to Roshi, a world-class martial arts master who was once the strongest man alive. He is also one of the very few sensei's who know how to manipulate ki, so he is able to develop the Kamehameha, the most classic DBZ technique there is (although it is more often associated with Goku, guess who taught it to him?).

At the beginning of Dragon Ball, Roshi lives alone with a turtle on a small private island. He lives as a hermit, separating himself from the distractions of society. However, he's far from pure-minded, as he's absolutely obsessed with pretty young girls. He first encounters Goku and Bulma while the youths search for the Dragon Ball that had washed up on his shore. He learns Goku is the adopted son of Son Gohan, his former student, and is impressed by Goku's ability to learn the Kamehameha after only seeing it once. So Roshi takes young Goku under his wing, teaching him the Turtle school of martial arts.

Roshi only rarely participates in the battles the Z-Senshi fight, but his home is always open for the group to assemble, whether for a social gathering or a strategic meeting. And despite his various perversions, he's respected and appreciated by his former students (although the various DBZ ladies keep their distance most of the time).