The opening of The Stranger begins with the death of Meursault's mother. We do not learn her name, her age, or even see her alive. Her existence as a character is almost solely as a distracted afterthought in Meursault's preoccupied mind. In spite of her portrayal as such, Maman and her death clearly represent what I believe to be the essential question of the story:
Is an existential philosophy inherently compatible with humanity?
Existentialism firstly challenges us to reject the norm of such constructs as God, family, and love, and embrace the absurdity of a life that is as random and inexplicable as we fear it to be. When discussing what it means to be human, such constructs as these are often top of the list. In rejecting these it would then stand to follow that in doing so we reject the principles of our own human nature.
In The Stranger when Meursault is arrested after murdering the Arab (murder often considered the gravest offense to humanity that exists) his lawyer asks him if he was sad about Maman's death, to which Meursault replies 'no'. In doing so he dismisses Meursault's humanity, in this case Meursault's ability to be repentant of and redeemed from his crime, due to his nonconformity to the socially normative version of grief the lawyer expects. What the lawyer fails to conceive of is that Meursault not grieving as he 'should' does not equate a lack of love for his mother, nor a lack of humanity.
This addresses a second (and more often forgotten) aspect of existentialism. That God, family, and love should be dismissed as constructs but not necessarily as concepts. These things exist and are true in face of the absurdist universe painted by existentialism. But they are not true because that is the order of the universe, as societal norms would have us believe. They are true because we choose to create them and recognize them despite our consciousness of the random universe. In this existential allows us not only more freedom, but a greater capacity for humanity.