What does aunt Alexandra think is the most important factor that defines a person in to kill a mockingbird?

Some people were said to inherit family traits. The Cunninghams could always be trusted, and the Ewells were always dishonest, according to the townsfolk. Aunt Alexandra wholeheartedly believes this. She tries to stop Scout and Jem playing with the Cunningham children because of their lack of 'background'. Aunt Alexandra also believed the Ewells to be 'trash'.

So, in answer to your question, Aunt Alexandra believed that background and family traits were the most important factor that defined a person.