I'm writing a concept for a comic book that takes place in three fictional East Asian kingdoms ( roughly corresponding to Japan, Korea, and China) in the 17th century. The two protagonists are of lowly birth but after encountering a specific character, they travel across the three kingdoms training to be great warriors like, if not better, than the warriors of the kingdoms armies. Now, if my knowledge of Japanese history is correct the samurais were members of the nobility just below the emperor and only they could be samurais. Despite taking some liberties with history (it is fictional after all), I still want to be accurate in my depictions. Any help? Thank you in advance.
Once we get to the 17th century, a samurai was not defined by his skills, but his ancestry and ties to his lord. Bushi were people who fought in the Sengoku and at the end decided to remain as warriors instead of becoming commoners (well it was against the law to become commoners towards the end, which also means some people tried, but anyway).
In the early 17th century, samurai might still have been heavily concerned with learning how to fight. But by the late 17th they were more bureaucrats than warriors. But no matter what a samurai's doing, a commoner wasn't defined as being less skilled than a samurai at what a samurai does, but by the fact he's not allowed to carry a sword, and is not employed in a samurai household as a warrior-servant.
In other words, he would have been a commoner. Maybe a farmer, a merchant, an artisan, a performer, but a commoner nonetheless.
Now if he was of great skills and/or had ties to a samurai, he might get adopted and/or raised up to a samurai, but before that he was either a warrior-servant, who were considered low class buke (warriors), or he was a commoner (or aristocrat or monk but I'm guessing that's not what you are asking).