Captain Zhu was the original captain of the Hui, the ship that brought Bar-Asher to Japan in The Accidental Samurai. Captain Zhu was accused of being a pirate and killed by those who claimed he owed them goods. So was Captain Zhu a real person?

No, but he is based on real people. In The Accidental Samurai it is clear that Captain Zhu and the Hui were involved in trade between the Kingdom of Silla (Taken over by the Kingdom of Goryeu by 964)  on the Korean Peninsula and the Chinese Port of Ningbo (Mingzhou at that time). This was an important trade route at that time and many merchants were engaged in trade. If a merchant was involved in what we might call today “shady dealings,” the line between merchant and a pirate was most likely in the eye of the beholder. The implication in the book is that Captain Zhu was a legitimate merchant who sometimes resorted to piracy or at least questionable dealings. There were many legitimate merchants and outright pirates operating at that time, and it is certainly possible that there was a grey area between the two extremes in which people like Captain Zhu fell. While it cost him his life, it also caused the Hui to be damaged and escape in a manner that led it accidentally to Japan.