If heroes cannot die because of the fairies' protection, how did Minowa Gin die?
In Gin's era, the Hero System wasn't fully developed yet. The girls didn't have fairies to protect them, so they could and often did get hurt. They also didn't have the sealing ritual or Mankai at first either, so in the early battles they had no way to kill a vertex; the best possible outcome was to drive it off without any of them sustaining any serious injuries, but they weren't always that lucky. Gin died protecting the other two girls after they had both been injured in a battle, but for the full story (which is both heroic and sad) you really should read the light novel - it's a good book, and answers several questions that were left open by the anime. All of the chapters have been fan-translated into English and posted online.