Unlike Mariya Shidou, she appears to actually be the kind, innocent girl that Mariya Shidou attempts to portray. She is also quick to get her feelings hurt, as well. The agreement for the competition was the loser would be chosen if someone found out from either school; Kanako Miyamae, thinking Shizu Shidou was Mariya Shidou, mentioned Mariya Shidou's touching her, which revealed to Shizu Shidou that Kanako Miyamae knew Mariya Shidou's gender, thus defining Mariya Shidou as the loser.
Shizu Shidou had only went into the school to switch with Mariya Shidou for their schools' physical examinations. However, because Kanako Miyamae found out Mariya Shidou's secret before she began attending the school, it was null, and because Kanako Miyamae knew Shizu Shidou's true gender, ordinarily, Mariya Shidou would've been claimed victor. But out of generosity, Mariya Shidou chose to overlook it in favor of his twin sister.
The only way to distinguish between Shizu Shidou and Mariya Shidou is by the mole under their eyes; Mariya Shidou's is under his left eye, while Shizu Shidou's is under her right eye. Because of the switching with her brother her original name was Mariya Shidou instead. She is 160.3 cm tall, which makes her marginally taller than Mariya Shidou, and happens to be a sore point for him. The two siblings regularly compete, not just in height and weight, but in studies as well.