The one that lived next door. They were going to go to college together after she returned from Europe. Did I miss something, or did she just stay in Europe?

Jean Pearson (Cynthia Pepper) does return for just 1 episode, in Season 5 (Ep. 9, Nov. 1964 "Goodbye Again"). She wants to hook up with Mike again, but by that time Mike is engaged to Sally Morrison (Meredith MacRae).

In that ep, there's no mention as to exactly where Jean has been; she's in Bryant Park "visiting [her] grandmother for a few days." She claims it's been "four years" since she lived next door, and again saying "almost four years" since she last saw Mike. (Both Steve and Mike repeat "four years.") So at least the time gap since their HS "grad night" is well-acknowledged.

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In real life, Cynthia had left M3S to star in the short-lived TV series Margie. Apparently the M3S writers never saw any need to fill-in info on Jean's absence.

She's here...
Season five, episode nine.