Yajirobe is a human who first appeared near the end of Dragon Ball. He is a samurai who used to live in the wilderness. He met Son Goku when Goku ate the fish he was cooking, which lead to a conflict between them. The two become friends just as Piccolo Daimao's third offspring Cymbal shows up wanting the One-Star Dragon Ball Yajirobe has. Yajirobe defeats and eats Cymbal. With the Piccolo Daimao threat abound, Yajirobe cowers instead of joining Goku to defeat him. He supports the Z Fighters mostly from the sidelines after by living with Karin, growing and delivering Senzu to the group.
Yajirobe shows a surprising moment of bravery during the Vegeta Saga, when he uses his katana to sever Oozaru Vegeta's tail, and later slicing through Vegeta's armor.
After that, his role as a fighter ends, and his overall role in the series is just giving the heroes Senzu Beans, although he is the spotlight of some comic-relief filler during the Artificial Human and Cell Sagas. By the time of the Majin Buu Saga, he is a minor character, but still makes small appearances. He only briefly appears in Dragon Ball GT, now having a mustache, in episode 40.
In the alternate Future Trunks time line, Yajirobe, interestingly enough, didn't go into hiding with Master Roshi and the others but instead fought against the Artificial Humans. Although this is not seen in the Trunks TV Special, a brief scene of his death is seen in the Cell Saga of the anime.
He is voiced in the original Japanese version by Mayumi Tanaka, who also voices Kuririn (this was more than likely done because the original manga has Goku saying that Yajirobe's voice sounds like Kuririn's).