With Blue appearing in the first five gens, being mentioned to have been in Kalos (presumably he left Kalos for Unova to participate in the PWT as B2W2 and XY share the same part of the generic timeline). And reappearing in Gen VII alongside Red (Red is also a respected Champion, but he's also the original MC, so that might have more to do with it).
While Steven appears in every gen except for Seven. The guy even appeared in HgSs and was mentioned to have been in Sinnoh in Pt.
While Cynthia... well you know her. She's been in every Gen since her intro except for Six which did a great job at not bringing back old characters. In fact, Cynthia appeared in every Gen IV-V game!