To be honest, I got both by sheer luck. So, it is possible but a small chance. I advice to be cautious about the color behind the girls, since there's a rarity chain. I believe that yellow is supposed to be an epic girl, so if you see those you might consider to be wise about using koban. Too many already complain about the droprate, but it's still fair. I've had my fair share of events with no girls dropping, so that's the fair part. if you got every girl at every event, then there wouldn't be a point in trying, in my opinion.
Yes, I get that it's frustrating. But the truth is, which no one will like but I want this part to be included and clear, that if YOU decide to use kobans then it's on you. They don't increase chance, just the ability to play more in succession. I personally only use them for Epic P 10x.
And sorry for when the thing about having the girls stings, I'm just honest and trying to help clear things up. I love Hentai Heroes and play it as much as I can, with that I only hope for it to get better.