Chekov's Gun.
For it work, the plot device and or character needs to be present in some form, appearing insignificant until they are officially used and or acknowledged as crucial or at least relevant.
Of course, if the Chekov's Gun did not appear prior in any way then it's not Chekov's Gun but a retcon known as Remember the New Guy?
Remember the New Guy differs with how the plot device has NOT appeared at all or alluded to and yet has been treated as if they always had been around.
A Doting Sensei has a Chekov's Gun in one of the original five chapters.
Like the original chapters, this plot device wasn't supposed to mean anything in the grand scheme of things. It had some implications, but it wasn't part of the bigger picture.
But just because it wasn't doesn't mean it can't.