|Occupation||Trainer and Manager|
|Manga Debut||Round 327|
|Anime Debut||Round 5 (New Challenger)|
|Voice Actor||Omoro Masayuki|
Bill Martin is Ricardo Martinez's head trainer and manager. He is a world renowned trainer that had trained up to 7 World Champions (including Ricardo).
As of him, he appeared with Ricardo when they came to Japan, mostly in the sparring match against Ippo. In Ippo's spar with Ricardo, Bill was quick to decipher Ippo's style and was surprised, but not worried when Ippo showed his Dempsey Roll, being amazed that someone from Japan was capable of using it. As of being such trainer, within also had trained in the past, six world champions and then Ricardo who is the seventh, means he is really an experienced trainer. His history with Ricardo is still unknown at this time, as of how Ricardo and him met, and went all the way to the top. But at some point in the past, he might had been a professional boxer. Ricardo refers to Bill by his first name.
Bill has the appearance of a middle-aged man. The hair, and his appearance is simple, but mostly Bill appears with his glasses in Ricardo's matches so far, and later in Ricardo's mansion with the same appearance too. Until now he did not appear without them
Bill is somewhat of a cocky and intelligent trainer, just judging by the fact that when Ricardo was going up against Eiji Date he wasn't worried much. However when Ricardo hesitated to finish Date due to the fact that an unfortunate accident could occur, he demanded that Ricardo finished him.
- His surname was originally Bill Stewart in the anime and manga, but was changed to Bill Martin in Round 1122 (both raw and translated) for unknown reasons, unless it is his middle name than his surname.