This is what is said in the Analysis post "Update: According to the press release via DenFamiNicoGamer, Shez runs into Byleth in the imperial year 1179, a year before Three Houses begins. He/she then begins wandering Fodlan, honing their skills for a rematch. Eventually, they bump into the students chased by bandits (so it’s 99% the same event as Byleth originally meeting the students).
Update 3: Also in the press release, but I missed it the first time. As pointed out by 黒凧 BlackKite via Twitter, Shez will choose to join the Black Eagles, Blue Lions or Golden Deer as a student. So their role won’t be completely the same as Byleth. (It makes sense since Shez is probably completely unknown to Garreg Mach, whereas Byleth at least had Jeralt as a common link.)"