Born the same as her older sister, she grew up in a dark family background. Yeri discovered her ability to shift herself as a weapon when she was five years old, the first weapon that she shifted into is a shotgun, which her mother got surprised about her ability but it never surprised their father. She starts training her but ended up running away from the trainings because she was not strong enough to be a hero. 

Yeri reads a lot of superhero comic books as she grows up, Taeyeon always tells her about the stories of their mom as superhero. The younger one looked up to her mother as she is her inspiration to right fictional stories and also boosting her self-esteem. But that was not enough for her to be confident as she was also bullied by some students in the Academy because of their father. Her sister always there for her and needs to be there as she doesn't want her to be astray like the other students that turned into a villain. When Yeri discovered that their loving mother died because of their father, instead of getting angry towards him, she regret that she was not there to protect her. That made her more quiet and distant to everyone except her sister who was very protective of her.

 

But those negativity won't be an obstacle to her will to be a hero, she has her own way to be a hero. Somehow, she could still talk to a few friends and shows her jolly and bright side despite of what had they been through these past few years.

 

Even though she and her sister were ten years away from their age, she could easily keep up with her sister who is now a teacher in Sora Academy. Taeyeon and Yeri were practicing together whenever they have free time or sometimes after their class. Yeri knows what her sister is doing, she wants her to be a hero that she wanted but she has no courage of being independent. She lacks confidence and strength but not her will. Since she has been used with her power, she is not ready to be a hero yet. It takes millions of speech before she was convinced to be a hero. When she was enrolled in Sora Academy, she showed excellent work in their first exam. Yeri named herself as the Catalyst, the name that she will be as a future hero as she can be both a weapon and a user. Despite of all the hate that other people think of the sisters, Yeri is slowly keeping her stance straight and not to be affected by their opinions about their family. She is a type of a person she never anger nor mad, she is just disappointed or upset. Yeri is opposite to her sister who is vengeful and only has her one goal in life: to defeat their father, the Stygian. Anyone could admit that Yeri is too innocent of being a hero but she is not, she has seen heroes and villains fighting in the streets with her sister and always takes note of their information so that she knows where to attack. She might not be confident, she is a smart kid. Her ability to transform into a weapon is limited, she could only do weapons that is made of  iron and steel like revolvers and a halberd. Whenever they are losing from the villains, they will form a formation with her sister. They called it, "Final Phase". As Yeri will become a complete form of Death Scythe Scimatar while her sister is merging her power with her to get stronger. They did this once in their training but never again due to Yeri's limited energy and stamina, so they call it off and only doing it when it is very necessary.