I first read the book about 6 years ago when I was in the 5th grade and I didn't understand ANYTHING about the relationship between Evelyn and Younger Brother (obviously) but even then she was my favorite character in the book because I understood that she was helping Tateh and his little girl, and she was trying to escape her life the best she could.
When I listened to the musical and played Evelyn in a production I did of Ragtime, the character made a LOT more sense I think. I couldn't find much about Evelyn Nesbit in my research as to her personality, but I don't think she would have really chilled with the immigrants.
She's on long enough to motivate Younger Brother and that's good enough. I've seen Evelyn's who were terrible but it didn't matter because they weren't on long, and I've seen Evelyn's who were really imprinting. They weren't on for long and yet at the end of the show I remembered them really well. If played right, she's unforgettable.
"It's a great feeling of power to be naked in front of people. We're happy to watch actual incredible graphic violence and gore, but as soon as somebody's naked it seems like the public goes a bit bananas about the whole thing."