She was the little sister no one thought about and he was the perfect child that could do no wrong. That is merely the way things went at the Yagami household; Light is placed on a pedestal and Sayu is left in the low light. She should hate him; she knows that; she doesn't though, Sayu honestly looks up to her brother. After all, he unknowingly helped her become what she is today.
A little known fact among anybody is Sayu's life is this; Sayu is top of her class. It's been that way since her parents crushing disappointment when she barely made it into the third grade. She never wanted to see them go through that again, so she got her act together and began to pay attention in class. Her parents don't know that though, because neither Light nor Sayu are ever forced to show their grades to their parents.
'If they think I'm just an idiot, why try to change things. If a small fact like that changes the way they see me, then they never loved me to begin with,'
That's how Sayu saw things, anyway.
"Sayu, could you go tell Light breakfast is ready whenever he is?"
"Sure thing mom, no problem," Sayu replied, artfully pulling off her act of stupidity once again, "Give me a sec..."
Not ever Sayu knows why she does it, but somewhere deep inside herself she knows that one super genius child was enough for the family; she knew telling her parents anything would upset the status quo of the household. Her parents would treat her differently based on the fact that she was yet another, 'promising, young intellect.'
That term had always irked Sayu like no other; not only were they saying that young people couldn't be smart, but they were practically saying that every intelligent person in the world had to amount to something amazing in life.
'What if I don't want an amazing life,' Sayu would ask herself over and over, 'What if I want to be a small shop manager with an average husband and nice, happy, normal children?"'
"Oh, Sayu?" Her brother's voice said as they bumped into each other on the stairs, "Coming to tell me breakfast is ready?"
That was another thing Sayu hated; how did Light know everything all of the time?
"Uh, yeah," She laughed, noting that the words that fell from his lips were rehearsed and planned, "Tell mom I'll be back down in a minute, please."