When their parents were still alive, it was Mako who was the funny guy and the one who could wear his heart on his sleeves, while Bolin was more aloof, shy, and well-guarded, feeling a little inadequate compared to his firebending brother (at least until he became a man, grew some really large balls, and took on the reins from Lin Beifong as Chief of Republic City). When their parents died, however, they switched roles to cope with their loss and to comfort the other brother. Mako become more aloof and well-guarded while Bolin became the funny guy and the one who wore his heart on his sleeves. As they grew older, both of them would look at one another from time-to-time when the other one wasn’t paying attention, and remember how they were when they were kids, with Mako smiling at the memories he had of those times when he was just like Bolin and how happy he was, and Bolin looking sadly at the same memories, remembering how inferior he felt compared to his older brother and how much he needed to compensate, and hide his pain, by being even funnier.
After being prosecuted for illegally destroying parking tickets using firebending, Sergeant Mako was forced to do 1000 hours of community service. However, because he was a police officer, and the Chief’s older brother, his community service would be done in secret in order so that scandal wouldn’t be brought upon the whole police force, including the chief, himself. Chief Bolin decided to let him do some covert community service in the guise of a “police outreach” program that included reaching out to the impoverished. Since he was well adept at taking care of children, Chief Bolin decided to let him focus on reaching out to little kids who were living on the streets. Sergeant Mako was entirely against this, finding the act of taking care of children to be too stressful for him, so he decided instead to just cook for the little kids and bring food to them, hoping as they ate that they wouldn’t bother him too much while he waited for his hour-a-day community service to wound down. He would cook them cupcakes or danish, whatever type of bread-like food that would make it hard for them to speak as he waited for the hour end. After a while, in a twist of fate, the children started to endear themselves to him, and vice-versa, and they began to affectionately call him “Mama Mako”.
At first, Sergeant Mako found this to be really sweet, remembering how his younger brother called him the same name when they were younger, however, when the whole police force found out about the Sergeant’s nickname and decided to start using that as his call-sign whenever they were on a mission, Mama Mako, oh, *ahem*, excuse me, Sergeant Mako became angry. He immediately went to the Chief and asked that he punish anyone, past, present, and future, for calling him this embarrassing nickname. Chief Bolin agreed to this request and forbade everyone in the force from ever calling him that name again. Sergeant Mako thanked his brother, happy that his younger brother still had his back. Unbeknownst to him, however, it was Chief Bolin who shared to the whole Police Force that he used to call his brother, “Mama Mako”.