@NecroKnight If I may,
One possible way to make the character possible whilst retaining a good level of lore friendliness into the roleplay, you could say that she was trained under the School of the Cat, but found her moral alignments to not align with it. During her travels or when she became a full fledged witcher... She found notes which spoke of Merten and the School of the Manticore. She then sets out on a journey to join them and thus providing you with a possible solution.
One possible way to make the character possible whilst retaining a good level of lore friendliness into the roleplay, you could say that she was trained under the School of the Cat, but found her moral alignments to not align with it. During her travels or when she became a full fledged witcher... She found notes which spoke of Merten and the School of the Manticore. She then sets out on a journey to join them and thus providing you with a possible solution.