Yui is a girl that has loved music ever since she was a child but her overprotective mother prevented Yui from playing the drums, her favorite instrument. Later, when she was in Keima's body, she had the chance to play the drums like she wanted. At first she was quite uncomfortable being in a boy's body, but she started developing a more assertive, masculine personality after a few days, even changing her personal pronoun to "boku" (normally only boys refer to themselves as this).
Reason For Cross-Dressing
Yui was forced mentally into a man's body and now feels more comfortable wearing male clothing as well as male pronouns.