Son Goku is the main hero of the series, and is the major focus in many episodes. He is immediately recognizable to many people and is one of the most well known anime characters even today. Most people can identify him, and he's even been the icon of several products. This page will focus on his past, his design, and his personality.
Goku is a Saiyan, a human-like alien race whose home planet was destroyed by Frieza in a mass genocide. Goku is one of the few survivors of the explosion, as he was sent away from the planet as a baby by his father Bardock before it happened. After he landed on Earth he became the adoptive grandson of Grandpa Gohan, and his Saiyan name, Kakarot, was changed to the name he uses now, Goku. At first Goku was very aggressive towards Gohan, which is the nature of the Saiyans. However, this changed after he fell into a ravine and suffered a serious head injury, nearly killing him and giving him long-term amnesia. This made him forget 
his mission to destroy Earth, and completely forget his Saiyan heritage and nature, causing him to become a kind and mild-mannered young boy. Gohan nursed him back to health from the injury, and took care of the now passive Goku. However one day he discovered that because of Goku being a Saiyan, he will transform into a Great Ape, or Oozaru, at the sight of a full moon. Goku was told never to look at the full moon, but when he did, he transformed and unknowingly killed Gohan with this transformation. He thought Gohan died naturally, and was left to fend for himself, which started his grand adventures in Dragon Ball.
Goku is a cheerful and energetic character that tries to be kind to everyone, even his enemies. He often tries to give his enemies a second chance, which will usually come back to bite him, but sometimes works in his favor. He's been criticized for that on multiple occasions, as it has put both himself and others in danger often. Goku also has a large love for food, which has been made into a sort of comedic trait. He has a large amount of power and loyalty to his friends. Despite his large power, he isn't very smart when it comes to math, as he never had any schooling. This is evidenced in the way he speaks in the Japanese versions of the Dragon Ball series. Despite this, he can still come up with a good battle strategy, and be able to easily point out an enemy's weakness even after only a few minutes of fighting. He is also very 
determined, always wanting to push himself beyond his limits and test out new attacks, even if it means hurting himself in the process. Goku has been stated to be pure of heart, meaning that he has no negative feelings or thoughts. This plays along with his innocence and experiences, as he never met anyone except Gohan for a long time until he met Bulma. Despite this Goku may not have a sense of family, evidenced by the way he acts around his wife Chichi and his son Gohan. He seems to just think of them as very close companions rather than a son and a wife. But even if this is the case, he still has a strong sense of protection for his friends and family, and will try to protect them no matter what. However, if there is no danger, he has no problem with just leaving them on a whim to go train. He is often seen putting his training in above all else, as he has an overwhelming desire to become stronger. But, if danger pops up, he will immediately come back if his friends can't handle it. Goku loves fighting, and always wants to challenge strong opponents no matter what. Even if he's been warned several times not to like with Beerus and Frieza, he still will. And even after getting completely smashed by a strong opponent, the second he can stand back up he'll want to fight them again. This drive to fight has put both himself and his friends in danger, so others frequently protest against it. Currently, Goku is becoming less and less like your typical hero. Dragon Ball Super is starting to use Akira Toriyama's original vision for the character, which makes him less heroic. In previous anime depictions, Goku has been shown as a complete righteous hero, but Toriyama has stated that's not how he originally depicted Goku. Now in Super, Goku has been slightly demonized by other characters, due to his drive to fight strong characters. This caused him to talk to Zeno and set up a tournament where the losers are completely wiped out. Goku doesn't seem to care about this, and he even uses "but I want to fight strong guys" as an excuse for his actions. With all of these traits, Goku has started as a hero, but ever since the Universe Survival Arc has become more of a "chaotic neutral" character, sort of like an anti-hero. Despite this, he is still the main character, even though he may not be acting like a protagonist.
Goku's design, like many things in early Dragon Ball, was inspired by Journey to the West. His design was based on the Monkey King, Sun Wukong. So much so that the first draft of Goku's design was that of a small monkey, and it eventually evolved into a boy with a tail. His current design was based off of the main character from Toriyama's prototype manga Dragon Boy and Tongpoo, and from his other manga the 
Adventures of Tongpoo. Toriyama made three drafts, the first draft was the one mentioned before, of Goku being an actual monkey. The second draft was of a human dressed in sailor clothes that rode rode in a flight mecha instead of Goku's current flying Nimbus. The final draft was the draft we saw, which was of a boy with a monkey tail. As the design evolved Toriyama made Goku grow up to continue the series, which was a very uncommon thing to do in Shonen manga. Although it helped the series overall, and got more people interested in it because of the change, and allowed for a different story to occur. Now Goku is an adult, in about his forties, and the story, his design, and his character continues to develop as the series goes along, even more than it already has.