Krillin is Goku's oldest and best friend, who trained with him under Master Roshi and fought with him against many enemies. He was killed by Frieza, which triggered Goku's first transformation into a Super Saiyan.
Bulma is a genius inventor and the founder of Capsule Corporation, who met Goku when he was a child and helped him in his quest for the Dragon Balls. She is also the wife of Vegeta and the mother of Trunks and Bulla.
Piccolo is a Namekian warrior and the reincarnation of King Piccolo, who was once Goku's enemy but later became his ally and mentor. He fused with Nail and Kami to increase his power and sacrificed himself to save Gohan from Nappa. He also trained Gohan for the fight against the Saiyans and the androids.
Gohan is Goku's eldest son and a half-Saiyan, who inherited his father's courage and potential. He was the first to achieve Super Saiyan 2 and defeated Cell in his perfect form. He also became the Great Saiyaman and fought against Majin Buu and the Zamasu.
Vegeta is the prince of the Saiyans and Goku's rival and ally, who was initially a villain and tried to destroy the Earth. He later joined forces with Goku to fight against Frieza, the androids, Cell, Majin Buu, Beerus, and the Frieza Force. He also married Bulma and became the father of Trunks and Bulla.