As Ririka takes damage, her Health will deplete and her clothes will be destroyed - leaving her exposed and vulnerable to the tower denizens. Some enemies might be especially good at destroying clothes, so tread carefully!
Without clothes, monsters will try to assault her by grabbing and holding her into place. Ririka has a last chance to escape their perverted clutches if the player successfully inputs a random sequence of commands that are briefly displayed at that time. If she fails to escape, the attacking monster will proceed to do as it wishes with her.
But that's not necessarily the end of the line for Ririka! Being violated by monsters causes high damage, but if her Health isn't fully depleted yet she can still go and get her revenge... with the exception of Hardcore Mode, where there's no Health Bar and her clothes are all the protection that she'll get.
Not every enemy in the tower might be actively trying to assault Ririka themselves, but even those that aren't can still be problematic as they might leave her vulnerable for others. Bosses, in special, are particularly dangerous...