The very first episode of Dragon Ball Z opens with Goku's long-lost brother, Raditz, showing up and threatening his son, the planet and everything else he cares about. It's one heck of a way to open a series, both for Japan, which got all of Dragon Ball prior, and for America, for which this was the first foray into the DBZ franchise.
Raditz dies in the second episode of Dragon Ball Z, never making another major appearance in the series and thus becoming a passing memory; a character who faded into the past. However, there's much more to Raditz than just these first two episodes, and if you want to learn all there is to know about Goku's older, evil brother, we've got you covered.
Raditz is a Saiyan, a race of beings with naturally high power levels, among other attributes. Though Raditz is on the weaker side, his power level is far beyond anything Goku or his friends had encountered at the time of his arrival on Earth. On top of having an incredibly high power level for the time, Raditz is also capable of supersonic flight and shooting a variety of powerful energy blasts.
Additionally, Raditz has super strength — partially due to the Saiyans' home planet having 10x Earth's gravity — as well as incredible speed, great durability, high stamina/endurance and the ability to turn into a massive ape-like monster when looking at a full moon, though we never saw him do so.
Unlike Goku, who has used his incredible power to protect others and fight evil, Raditz is far more sinister. In fact, most Saiyans have vicious, sadistic, power/battle-hungry personalities, and Raditz is the best example of this: he's mean, he looks down on the weak and he wants to conquer Earth and convince his brother to embrace his Saiyan heritage and do the same.
Raditz is not above fighting dirty. After all, he kidnapped Goku's son -- his own nephew, Gohan -- in order to goad his brother into helping him take over the planet. Additionally, Raditz has little to no empathy and is quick to attack or kill anyone who stands in his way, making him the poster child for the harsh ways of Saiyan culture.
Raditz's personality actually plays into his purpose in the story. As previously mentioned, he is meant to be the representation of Saiyans and Saiyan culture, an introduction to the concept of Saiyans that prepares the characters, stories and viewers for the worse version of Saiyans, Nappa and Vegeta, who head to Earth following his dying transmission to them.
Additionally, Raditz serves as an expository device to tell Goku that he has been an alien this whole time, introducing the concept of Saiyans on top of being the standard representation of them. Furthermore, after telling us of the Saiyans, Raditz also informs us that Goku's original Saiyan name was "Kakarot" and that there are very few Saiyans left alive, following the destruction of their home planet.
OTHER APPEARANCES & TRIVIA
Though Raditz dies in the first episode of Dragon Ball, he does make a few other appearances in the franchise. In addition to making a brief appearance in Otherworld (Dragon Ball's afterlife) following his death, and being a playable character in a number of Dragon Ball video games, Raditz has also made appearances in the Dragon Ball Minus manga, parts of which were adapted into flashback scenes in Dragon Ball Super: Broly, which was the most recent time the character showed up. In the film, a young Raditz was seen on a mission with Prince Vegeta and Nappa, whom he would travel with for many years to come, eventually leading him to Earth to reunite with his younger brother.
Other interesting trivia about Raditz includes the fact that his name is a play on the word "Radish," since all Saiyans are named after Vegetables. Additionally, Raditz is one of the few villains in the series to have themed attacks; his techniques featured throughout the anime and video games are named after the weekend, like his Saturday Crush and Shining Friday attacks. Raditz is also unique as a villain, since he is one of the few series antagonists not to have some form of heroic or anti-heroic redemption.