Garlic Jr. is a demon best known for causing trouble in Dragon Ball Z: Dead Zone and later the Garlic Jr. Saga, a filler arc that takes place between the Freeza and Trunks sagas. His whole motivation stems from his father being locked away by Kami in the realm of darkness—something that left him bitter and obsessed with revenge. He collects the Dragon Balls to wish for immortality, which goes about as well as you'd expect.
In the movie, Gohan eventually defeats him by shoving him into the Dead Zone, an alternate dimension Garlic Jr. himself can control. Except he doesn't stay down. He manages to escape using the Makyo-sei, a power source he taps into to come back stronger, and he's armed with the Black Water Mist—a technique that turns anyone who breathes it into mindless demon slaves. The anime saga ends when Gohan destroys the Makyo-sei, draining Garlic Jr.'s power and trapping him in the Dead Zone for good.
Interestingly, Garlic Jr. is basically a callback to Pilaf, an earlier villain from the series. The two are strikingly similar—both are small, cunning dwarves with nearly identical designs, and both were voiced by Chuck Huber in English and Shigeru Chiba in Japanese. The big difference is tone. Pilaf was played for laughs as comic relief, whereas Garlic Jr. is genuinely menacing and sadistic, which perfectly mirrors how Dragon Ball itself became a much darker and more serious franchise over time.
Content compiled by AnimeList.moe from publicly available sources.