We all love it when the assistant proves to be a question mark for the Doctor to uncover. Just like Clara, Alex has died twice and lived to tell the tale. But with each death, Alex has also had a helping of time travel, making her the perfect companion shed be unfazed by anything the universe had to offer her. As a police officer, Alex has the courage to stand up to the bad guy; whether its a hostage situation or a fleet of Daleks. And if she happened to die at any point, it wouldnt matter: the Doctor could just do a quick hop in the TARDIS and pick her up from whatever time zone she landed in. Shes the number two choice for a companion and why? Well, as well as her affinity with time travel and her courageous personality, Alex fits the main criteria of a companion to be selflessly dedicated to maintaining good rather than evil.