If they aren't too important until later series, then just reintroduce them later on. If Minor Character 1 is at a party MC is at in book one and the two discuss their shared hatred of the job, then never see each other again until book 3, you don't want to put so much importance on the minor character 1 in that first meeting to the point the reader suspects their role is pertinent to that single plot line. Just, in book 3, have them greet each other and MC be like, "Oh, hey, I remember you from Jared's party a year ago. Yeah, who would've thought he was a spy for the Russian government? What a shame."
Because, you do want every book in a series to, in a way, stand on its own while still relating to one another. Just reintroduce the character when it matters.