Sentence:  Yet when both Regine and her <WSD>flatmate</WSD> are at home they tend to spend their time in their rooms, although Regine (and probably the <WSD>flatmate</WSD> too) tends to spend more time in the communal areas of the flat when the other is not at home. 
Meaning of the word marked with <WSD> tags:  A person with whom one shares a flat.