Arya is no lady. She is tom-boyish and likes fighting and adventure, clever and "too smart for her own good". She has always have good relation with anyone she came across, be it Syrio Forel, Gendry, Tywin Lannister, Yoren, Hotpie.
Jaqen respects her for freeing him, and returns the favor by killing three men. Which should end the debt he had to pay her, yet he goes out of length to give her a coin and to teach her the ways of Faceless Men. He gives her a second chance, after she kills Meryn Trant, and on many occasions saves her from Waif.
On the other hand, Waif just simply hates her from day 1, or may be its jealousy/competition.
So we see two sides of the coin, Jaqen being positive to her and Waif being Negative.
But I don't understand why, Jaqen already paid his debt, and being Faceless Men means having no emotion towards others, why does he goes out of length to help her?
And regarding Waif, Arya hasn't done anything to her, and she seems to hate Arya - Why?
And hate is an emotion. so she is not following the rules of the Faceless Men.