Going back a few days, when Castor decided to stay in the black forest where the Amazons live, he approached Sansa with a genuine desire to train.
Sansa is a powerful expert, she even surpasses Oliver, so for Castor, she is a great teacher and training partner, so Sansa being a warrior, agrees to train with him, just for the fact that he is a guardian of Andrew.
The Amazons at this time depend a lot on Andrew and having taken Zany as his wife, his bond with the Amazons is strong, so Sansa thought that helping Castor increase his strength would benefit the Amazons, since, if the guardian Andrew was stronger, he could protect Andrew better.
At first, Castor was very diligent and dedicated himself to training with her, but little by little he became braver in other aspects, leaving Sansa in an uncomfortable situation.
