Lady was Wilson's maid and was seen briefly in the episode Clueless.
Wilson was able to hire a maid with the money that he made from being an oncologist. Wilson was nice to her and it angered his wife Julie. When Julie had an affair and she and Wilson broke up, Wilson moved out. Julie fired Lady due to her close relationship with Wilson. Wilson offered to keep paying her salary but she insisted on working for the money and so Wilson told her to clean 221B Baker Street without telling House in advance. However, House soon needed a box containing his old chemistry set to help him diagnose a patient. He found Lady in his house and was afraid she had completely rearranged everything. House yelled at her because he thought the box had been in the closet and he couldn't find it there but she told House that she had seen it under the bed. He found it and thanked her.
It's clear that the misunderstanding was that Lady made a distinction between the words "box" and "chest". Although she's obviously a native Spanish speaker, House didn't even try to explain the problem in Spanish, even though he's very fluent.