The best pet to choose would depend on what you like the look of because they are all reasonably the same difficulty to level up (Kess being slightly easier), and evolve the same level, But you can buy all three so it doesn't really matter.
Gran specializes in defence and strength (not as much strength),
Ishkal specializes in luck and defence (not as much defence),
and Kess is an allrounder (everthing being exactly thye same).
I mean that she looses all realisticness of mobility and the wings go from angel-like to weird and unrealistic blue wings that wouldn't connect from the back properly. This okay?
Gran's 3rd evelution is good, not much different from it's previous evelution (2nd) but is obviously stronger build and has a larger and stronger beak, which I strangly liked.
Good first game, other 4 (or 5 including the 'saga') are better. my exp won't show on upgrade page, and I originally played/completed on gamefreak. so i'll have to do them all again!
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