Sparring with Argine is good for form, because what she lacks in instinct and raw talent is compensated by her intense precision. Sparring with Andrew is good for practice, because they're nearly the same size and spend generally the same amount of time at this sort of work.

Sparring with Rachel is good for innovation. She's smaller, and she's faster, and she's someone he cares about. It makes it harder to go for the legs while she's throwing the second blow.