Nothing ever really goes into that, unless it’s in Babylon, which isn’t translated, so...But Ishtar’s profile makes a point that her personality has been influenced by Rin so that the cruel part (i.e. the part that sent the bull after Gilgamesh rejected her) is suppressed and her good side is more prevalent. Also that, since she’s possessing Rin, a human, she doesn’t look down on humans like a god normally would.
And the description of how the relationship with the Master develops is: “She will come in contact with him in an affective and friendly manner, but just as 『a toy worth teasing』. But as their bonds become deeper, her natural disposition to take care of people and emotional weakness (she ends up empathizing with him) work against her, and he changes from 『a toy worth teasing』 into 『my precious contractor, whose future is worth watching over』.”