Plot related stuff happens with every character.Eh? Major/plot-related shit tends to happen whenever she's around though...
-Was there when Orion acquired his first G.o.D.
-Was there with Orion when they visited a former-temple-turned-test-site in Warthogs and he had a realization that he had been there (see: his first dream/nightmare)
-Invited Orion along for her family's Day of the Dead celebration, and during the trip he experienced his "first bleeding effect" and learned a thing or two about Luna's bloodline and the afterlife (which is likely going to come into play later on in the story)
The simple explanation is many people just thing she is cute and like her personality. She was not that high up among the main girls for me until the event where we went to her uncle's place. After that event she is definitely top 3 for me. That has to be my favorite event for any individual girl in the game.