Castor, unlike Andrew, is much more aggressive when pursuing women and this is because he was previously a captain in the Calci city gang, so his nature is more extroverted.
For this reason, from the moment he saw Sansa and was attracted to her, he wasted no time and went to talk to her. At first, Sansa believed that, as Andrew's guardian, Castor was only looking for a partner to train with, but she quickly realized that was not the case.
Certainly, Castor couldn't approach her right off the bat and declare his love for her when they didn't even know each other. Castor was initially attracted to Sansa's physical beauty, so knowing that she is a warrior he devised a simple plan.
He approached her and asked her to help him train. Sansa is much more powerful than Oliver and that was surprising enough, so it was not strange that Castor wanting to improve sought her guidance, so she accepted.
