The Scientist

Alex Karev and Paige Forbes Montgomery.

Paige Forbes Montgomery had a hard childhood ... her parents wanted her to be just like her older sister Addison Forbes Montgomery, become a surgeon and get married. However Paige decided to leave Connecticut and move to Seattle, where she met Meredith, Alex, George, Izzie, Cristina and later on Callie and Lexie grey.
Paige and Addison didn't have a good relationship seeing as they had a 11 year age gap.

Alex Karev also had a hard childhood after his father left, he had to step up looking after his mother and siblings.
Alex was in 17 different foster homes over the course of 5 years. He also spent time in juvenile detention for stealing food for his family. Sometime during his teenage years, he admitted his mother into a psychiatric facility.