i really think this story is quite flawed unless there is something I'm missing about Japanese culture, law, or the sho biz industry. but I think once someone literally assaults you physically and sexually I think the police and legal system will generally take your side.
First of all, sorry, but this reply will be quite big - if you don't want to read it all, I'll leave a few examples that made it to the news in the end.
Now, Japan suffers a lot from these problems that we are seeing in this story.
Celebreties in general live under a lot of pressure, both from their managers and their fans - they are basically dolls for everyone's entertainment and products for their companies/agencies. Some had to issue apology statements because it came to public attention that they had romantic relationships; "fans" will post indiscriminately on social media, sometimes calling for said celebreties to become targets of sexual violence because of their supposed "betrayal".
And the law... Even though there are, officially, legal mechanisms that in theory should cover cases of harassment and sexual assault, people (especially celebrities) are constantly discouraged, by their companias, the media, from using them - and even when they are used, the measures adopted do not punish the aggressors with the severity necessary to discourage future attacks.
As for the examples, the most notorious one I can mention is linked to the Love Live franchise, in which several "fans" launched an online hate campaign against the voice actress Suzuko Mimori, because she was getting married - they called out her supposed "betrayal", incited others to harass her in her social media accounts and postes photos burning Love Live merchandise in which she, or her character, were featured; some less notorious cases, these associated with internal harassment, came from the Takarazuka theater company - one case related to bullying ended with a young actress dead, and another case related to both sexual harassment and bullying ended with a stage director being fired, however, both had no further legal consequences.