Even after Misora uses her powers to reset the world back 3 days, Kei will retain those 3 days in his memory.
With a seven volume run of novels between 2009 and 2012, an anime adaptation was revealed in September 2016 for Sakurada Reset along with two live-action films.
With the show recently set for an April 5th, 2017 debut, more details have now arrived.
This means that all events and any memory of the past 3 days that “could have” happened, never happened.