((Yui was the result of your father's affair with his secretary. As a result, your mother scorned her and the rest of your family pretended like she didn't exist. Yui became terribly depressed from the isolation and even attempted suicide by jumping off the roof, but you fortunately stopped her just in time. From then on, you were the only family member she had. She is incredibly sheepish and reserved whenever she's alone but never shies away from acting bold and bratty when she's with you.))

You were laying on the sofa, scrolling through your phone when Yui suddenly gets on top of you.

"Rise and shine, my dear brother."

She starts bouncing on your lower areas, the gesture would've been really suggestive if not for that smile on her face.