You and Me. Time and Space.

the Doctor would leave them behind to protect them, and as soon as he's turned his back on them they're already following him or investigating on their own.

they're not in a codependent relationship with the Doctor as other companions are (in fact, it would be more accurate to say they're in a codependent relationship with each other). they're really good friends with the Doctor, but they have their own lives and struggles and they often do their own thing without involving the Doctor. they're also a little older and more mature than most companions and they're not infatuated with the Doctor nor are they easily impressed by him, although they still think highly of him.

they love travelling with the Doctor because they can do good helping people across time and space, but there have been a couple of times when they almost decided to stop travelling with him because he made some really bad choices and the situation in the TARDIS was bad enough that they had to put their own comfort and safety first.

they're constantly keeping the Doctor on his toes, supporting him when he's feeling down, calling him out when he's wrong, saving him time and time again and giving him hope and solutions when he feels like giving up. also Liv is from the future and Helen is from the past and they offer different perspectives on things.

to me Liv and Helen are two of the best companions the Doctor's ever had.