"Don't worry about it."
It's his brother's most commonly used phrase.
But despite whatever Mako says, his brother is one of the few things that Bolin does worry about. He's aware that his brother looks out and takes care of him at all costs, even at his own expense. He thinks that Bolin can't tell that when he rations out their meals, Bolin receives a significantly larger portion, leaving Mako with only a meager amount of food scraps. Bolin points this out to Mako one night, and he simply shakes his head and huffs, "don't worry about it, bro. Eat up."
He never did gain back all of the weight he lost on the streets.
And then there's the way Mako would attempt to mask sickness. When he starts to come down with an illness, he trudges along with a dry, hacking cough or a high fever, but when Bolin would suggest that they get help, Mako tells him curtly, "it's nothing. Don't worry about it."
But then he would always work himself to exhaustion, and his body would give out; he passes out while he and Bolin walk from odd job to odd job, in alleyways, in the middle of the streets. Anywhere, at random.
Every time, Bolin sets his unconscious brother down in an alleyway on top of his thin blanket, one of the only comforts the boy has, and wets his jacket with murky water from a nearby puddle, the only water source available to him, and places it over his brother's burning forehead.
And at those times, when he is exposed to the elements of the city in nothing more than his undershirt and thin pair of pants and his brother is out cold with a sweltering fever, all that there was left for Bolin to do was "worry about it."