In BTHS historical fiction, Mr Martinez might be a key figure in the historical events that the story is based on. He could be a person who has some influence on the main characters, for example, if it's about a school's history, he could be a principal who made important decisions during a particular era. Or he could be an ordinary person whose life story is used to illustrate the broader historical context of the time.
There's not enough information to say exactly who Kathy is. She could be an ordinary person with an interesting life story, or she might be someone known in a specific community or field.
Chen Kaixi was an assistant director. His representative works included the short film, The Clock That Was Two Minutes Slow. In addition, Chen Kaixi also played a role in the TV series Dear Enemy, played by Chen Yanxi. There was no other information or background information about Chen Kaixi.
Mr Martinez might play the role of a mentor. In historical fictions set in a school like BTHS, he could be guiding the younger characters, sharing his wisdom and experiences from the past. This would not only help the characters grow but also provide insights into the historical context. For example, if the story is set during a time of social unrest, he could mentor students on how to navigate those difficult times based on his own past experiences.
I don't have enough information to say exactly who Cap'n Kathy is. She could be an original character created by the fan fiction writer. Maybe she's a pirate - the 'cap'n' part could suggest that - who has some connection to the wizarding world of Harry Potter.
Kathy Andrews could be an ordinary person around whom some interesting stories are woven. Maybe she's a character in a local folklore or a self - published collection of stories. Without reading the actual stories, it's difficult to say more about her.