So I seem to remember YandereDev mentioning "sanity effects" for Senpai at one point. Now if this is something thats going to be in the game, in theory it means that there are two rivals that you can only deal with in a non-lethal way without damaging Senpai severally. (This is based on what we know about each of the rivals at the time of writing this) Those would be Osana, being the childhood friend of Senpai and Hanako, being his sister.