This is one of two articles on this topic that I know of. Here’s the second, also by TokyoReporter.com:

https://www.tokyoreporter.com/2018/04/13/kumamoto-boy-19-arrested-for-stabbing-prostitute-at-hotel/

According the second article, the victim is a sex worker.

– Kirk

https://www.tokyoreporter.com/2018/04/12/kumamoto-boy-apprehended-after-woman-28-found-stabbed-at-hotel/