Kate quickly realises after speaking with her father that there are two very different sides to Marlow, the man her father had intended her to marry. With him, he has been impudent to the extreme, whereas with her he has been all modesty and bashfulness. Both of them determine that she should not marry a Marlow who is either so dull and modest that he cannot even look at her or a Marlow who is so arrogant and rude that he will not be able to get on with Mr. Hardcastle. Thus it is that Kate plots to disguise herself as a servingmaid to get to the bottom of his personality. Note how she describes her reasoning to her maid in Act III:
In the first place, I shall be seen, and that is no small advantage to a girl who brings her face to market. Then I shall perhaps make an acquaintance, and that's no small victory gained over one who never addresses any but the wildest of her sex. But my chief aim is to take my gentleman off his guard, and like an invisible champion of romance, examine the giant's force before I offer to combat.
Kate therefore aims to "conquer" Marlow by adopting a very different personal herself. By disguising herself as a woman from a very different social class she hopes to have her beauty spotted by Marlow, but also to get to know him in a way that it is clearly impossible for her to do as herself. The diguise therefore is the key element to her strategy: she is able to befriend Marlow and to discover his real self, and if she is able to make him love her as a servingmaid, she knows she will have won him in realty, as his declaration in Act V demonstrates.