Gohan & Piccolo
Son Gohan
Son Gohan is Goku's first son. At the end of the series he was 26 years old but in this picture he is 9. For a short while after the second time Goku died in the series, Gohan became the primary character. Gohan is the most powerful non-fused character in the DBZ world, but lacks the love of fighting his father has and thus rarely uses his full strength. Gohan is a very good-natured boy and unlike his father, hates fighting, although for a while he was more powerful. He is very polite towards just about everyone except to his enemies, and when he's pissed off. Gohan is very bright and studies very hard, although this is somewhat due to the wishes of his very strict mother... Gohan is the first character in the series to have gone Super Saiyajin-2, during the Cell Game; the power of his anger was kindled by watching his friends get beaten up, and the death of Artificial Human #16 sent him over the edge. It was roughly at this time that the series switched main characters from Goku to Gohan.
Piccolo
Piccolo is Goku's old rival. He was born after Goku killed his father when he was a kid; in his last breath the old Piccolo spat out an egg, and the new Piccolo hatched. Piccolo then went on to fight Goku in the final round of the Tenkaichi-Budokai, which he subsequently lost by a very close margin.
Piccolo is a native of the planet Nameck, although he doesn't learn this for quite some time after his first appearance. As a Nameck, he can regenerate almost any part of his body lost. (usually the arms) He can also extend the range of his arms till they're really long. Later, he goes to Nameck to help Kulilin, Goku, Gohan, and Vegita fight Freezer. After the Tenkaichi-Budokai, Piccolo becomes more and more of a "good guy." Eventually he becomes Goku's friend; fairly recently he got pissed off at Trunks and Goten for calling Goku weak. Once one of the most powerful characters in the series, he has sort of reached a "middle ground," neither weak nor strong among the Z fighters.