In Ar Tonelico 2: Melody of Metafalica, in the talk topic Mind Guardian? (Lv 3) when Croix asks if Mir has a Mind Guardian she says at one point
You are just unaware of her. You must have met her in my Cosmosphere by now
However after her "story" finishes Ayatane introduces himself as Mir's Guardian (he doesn't say Mind Guardian though).
Now according to this GameFAQs guide, the requirement to unlock this talk topic is
After completing Jacqli's Cosmosphere Lv 3
but I haven't seen anyone who could be a Mind Guardian but Ayatane. So who is Mir's Mind Guardian and where would Croix have met her before?