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Michelle is Lily's high school friend.
In Sandcastles in the Sand, the gang discusses how being around someone from your past can cause you to revert back to the person you were when you knew them, a phenomenon Marshall dubs "revertigo." However, Michelle, who is completing her dissertation on behavioral psychology at Columbia, says it's called associative regression. Marshall and Ted tell Robin that Lily suffers from this more than anyone else; to prove it, they have Lily invite her high school friend, Michelle, to hang out with them.
Michelle arrives at MacLaren's, and Lily immediately begins speaking in the most unrealistic but highly amusing urban style imaginable.
Later, it's revealed that Michelle, is almost done with her dissertation to get her Ph.D. in behavioral psychology.