It's due to a combination of Her being his Soul , and Him being a Master of a Magic Type (or the Master of a PoP, not exactly sure which but I'm learning to him just being just Classified as a Master). And possibly because Alma was Summoned at the Great Seal, if it is due to her being able to Cast his Spells because he is also the Master of the MV? If it's because of her being his Soul Familiar (and so a missing part of his Soul now reunited) and because she was Summoned at the Great Seal and him being the Master of it then it's unlikely that she will be able to Cast any other type of Magic of Richters other then his Settlement Spells and the PoPs Element type Spells. Although if their Psi/Mind Bond increased to a high enough Level then it might be possible (that might be how she Cast the Earth Shield when she wasn't supposed to be able to?, she might not have been known it was possible till she really needed to do so?)? If it's due to them being Soul Pairs and with him being classified as a Master of Magics (even if just because of the PoP) then whenever he gets to a high enough Rank spin a Magic type she might be able to start Casting it as well. My theory is that he has to get to at Least Journeyman Rank, the Rank where he can Teach others Magic? Not really sure which is the one that is needed, time will tell. But if their Psi Bond can also allow the Sharing of Magic Skills and Spells then in time it might allow for Alma to actually Learn other Skills that Richter knows, she may become, Limitless!