Does Aífe even have the qualifications to be a Caster?
Could she maybe be Scáthach's daughter Uathach?
From my understanding about Aífe, she was known for being deceived by Cu Chulainn when he claimed her prized chariot fell off a cliff, so I'd see her as a Rider. On the other hand, Uathach doesn't really have much in the way of info other than being one of Cu's lovers after a dispute, so I don't see how she can be a Caster.