I've got Shehera after 8 days from the start of the event and until now i did not get the chance to drop Rumiko. I have crushed the ninja spy for days for nothing. I've sent a message to the developers and this is the answer

"Thank you for your message.
Although, I think that the drop rate is quite good, and I see that you managed to obtain Spring Shehera
The daily missions are also providing you with a great amount of kobans!"

I've asked for a slow passive to harvest kobans, like daily gifts for the login. I think this is not an answer to my question. What do you think?