Kamen Rider Grease made his debut in said film and the first thing he did was steal Kamen Rider Build's Phoenix and Robot Full Bottles. A mysterious Kamen Rider who seems to be from Hokuto, his identity and goals will be revealed in the TV series.
As of the moment, the only thing we know about the this Kamen Rider is that he transforms using the Scrush Driver and the Robot Scrush Jelly. He will likely debut in the TV series in January 2018.
[Source: Toku Connect]