http://www.amazon.com/People-Who-Eat-Darkness-Tokyo/dp/0374230595
There's a whole market for foreigners working as hosts/hostesses. Why do you think it's not legal to be a host/hostess as a foreigner? Maybe not as your main job, but I doubt it's explicitly illegal, and even if it is it's not like that's stopped the prostitution trade.
I'm currently working my way through People Who Eat Darkness by Richard Lloyd Parry. Parry is an editor of The Times of London, based in Japan, who recounts the murder of Lucie Blackman, a British hostess who mysteriously vanished from Tokyo. The book gives you a better idea of the roles of a hostess, life as a "gaijin," and the attitude of police investigations into crimes involving foreign victims. There's some Yakuza mentioned in this non-fiction, but it's mainly factual details about the investigation into finding her murderer. This story happened in 2000; book was published in 2011.
If anyone wants to read a true crime book about the way the Japanese judicial system works especially with cases involving murder and rape I highly suggest reading:
https://www.amazon.com/People-Who-Eat-Darkness-Tokyo/dp/0374230595
From the title, I thought it was Joji Obara, who killed several night club hostesses in the 80s through 2000 in Toyko neighborhood of Roppongi.
https://en.wikipedia.org/wiki/Joji_Obara
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There is a book that details him called "People Who Eat Darkness" that discusses his murder of Lucie Blackmann and other hostesses.
https://www.amazon.com/People-Who-Eat-Darkness-Tokyo/dp/0374230595
Not exactly a serial killer story, but People Who Eat Darkness covers the disappearance and murder of a British woman in Tokyo. It's eventually revealed that the murderer is a serial rapist, and a fair amount of time is devoted to his background and thinking.
The Devil in the White City covers one of the first modern serial killers in Chicago, and is being made into a movie.