- In Naisho No Tsubomi it turns out that Saya really is Tsubomi's unborn sister who came to the past to tell Tsubomi she loves her (and give her and her friends some sex ed for whatever reason). But why did she felt the necessity to go back in time to tell her that, if she would have plenty of possibilities to do so in the future. Unless, of course, something terrible would happen to either Saya or Tsubomi...
- This is only assuming one possible explanation of Saya's nature. Neither manga nor anime ever gives an explanation to who or what Saya is, except that she is what Tsubomi's unborn sister will be at Tsubomi's age. Whether this is astral projection, time travel, or something else is left open. Some events suggest that Saya isn't a real, separate entity from the future but truly the unborn child; for instance, no one is ever able to find Saya's home except to know that it's close to Tsubomi's, Saya says that there's no way to visit her home or call her there, and Saya and Tsubomi share a dream of Saya being nearly swept away in a river at the very time that the unborn Saya is under severe risk of being miscarried.
Fridge / Naisho No Tsubomi