Important character traits (character personality traits), are introduced either directly or indirectly when the character is first introduced. This important element of characterization lays the character's psychological groundwork and the basis for judging whether the character is acting consistently or inconsistently. Sometimes an author will have a character behave inconsistently as part of character and plot development (other times, a character may behave inconsistently simply because of a flaws and weaknesses in characterization).
When Marcus is introduced as the first-person narrator, he is just awakening in a room with a permanently leaky roof that invariably lets rain waters in thus disturbing his sleep. We follow him through his morning routine, out into the kitchen where he prepares coffee for his mother and through a laughing conversation with her. There are several things Marcus does in this introduction that indicate he is a supportive character.
To start with, the first action he takes upon arising is to empty one of the full containers catching rainwater dripping from the ceiling. Why is this significant? Some other youths, being tired and annoyed that they were being so unpleasantly disturbed, would have a loud fit blaming their mother and demanding she come take care of the water-brimmed pots and pans. They would have been unsupportive. That Marcus quietly goes about a routine of taking care of the rainwater so as not to disturb his mother shows that he is very supportive.
It was dark, but there was enough light coming around the curtains to let me slowly creep around the room. I stooped down to check one of the pots. I was almost filled. I picked it up ...
One more indicator that Marcus is supportive is brought out in his discussion of the state of the kitchen, a state matching the state of the whole house. Instead of being bitter and angry when faced with the reality of the house's condition, like dripping ceilings and faucets and warped wood, he thinks of the psychological advantages the rickety house offers him, like residence on the military base amongst others who have family away at the Afghanistan war.
The best early indicator that Marcus is supportive is the agreements and jovial moments he has with his mother. He is mindful of her worry and dedicated to not adding to that worry. They both are suffering during their father or husband's absence and both know the other is suffering. Marcus is dedicated to helping deflect an increase to that worry by kindnesses, like making coffee--after convincing her it was necessary for him--by telling little lies and accepting from her little lies about how they were sleeping and feeling, and by making her laugh. This is the most important way that Walters draws Marcus as a character who is supportive because sometimes adversity actually drives people away from each other instead of driving them supportively together.
"I smell coffee."
I turned a round. My mother was standing in the doorway, wrapped in her dressing gown.
"It's almost ready."
"Then how about I pour us both a cup and get our engines started?"
This was part of our morning routine. I put on the coffee and she poured us each a cup.