Goku is the main character in the Dragon Ball origins story. He's the one the story mainly follows from his early days. Also, Grandpa Gohan is important as he found and raised Goku. And Bulma is another main character. She's the one who introduced Goku to the concept of the Dragon Balls.
Well, the main character is definitely Goku. He's a Saiyan who grows up on Earth. His journey is at the heart of the origins story. Then there's Bulma. She's a smart girl who is on a quest to find the Dragon Balls. She meets Goku and they start their adventures together. Another character is Grandpa Gohan. He's the one who took care of Goku when he was a baby. Without him, Goku might not have survived and the story would be very different.