How did you come to the conclusion that Liz was the second girl? Lol. She wasn’t even in Part 1. Suguha has always been the second girl, even if she had fewer scenes in Part 2 due to the introduction of a bunch of new characters.
I honestly don’t understand why people thought anything good would happen to Liz while she was so close to Inoda. It was obvious things would end badly for her, especially with how she treated him. Suguha, on the other hand, was initially on the cliché NTR path until Fujino completely flipped her story and took it in a very different direction.
I won’t rehash all of Suguha’s development in Part 2, but she’s consistently been shaped into something unique over half the game now. She’s not like the other girls, because; what’s broken can’t be broken again.