People seem to forget that Elizabeth was one of the first companions in video game history where she wasn't this immediate liability that you had to protect harder than protecting yourself. It was a excellent gameplay choice to make her invincible, so you could focus on Booker's own health and the enemies, but she was also useful, tossing you items and finding rifts to open. Even her interactions made it felt like she was a proper companion and not this misogynistic damsel-in-distress stereotype.