Yuri wanted to be strong, she wanted to be stronger than her older sister.
Yuri knew that to be strong, she needed to work hard, and she did work hard. She spent all her time just thinking about becoming stronger, and she had nothing else to in her mind.
Being friends with Yumi, Devin, and others was just so she could have powerful friends with her.
The reason Yuri built a special friendship with Yumi was that She needed Yumi to perform the fusion technique. Otherwise, she would only take Yumi as a friend.
If Devin or any other S-Class didn't have the potential to be strong or didn't belong to a powerful family, then Yuri wouldn't even look at them.
This was how Yuri was. She was thinking about herself, how could she make herself stronger and stronger without resorting to nasty and dark ways.
This was why she used up all her strength and pushed her body to the limit to learn the second ability of her Angel's Daughter ability.
