El Capitan may grow into a loving brother, but that's due—in large part, at least—to the loving guidance of Helmud. Weird, repetitive, oddly bloodthirsty Helmud.
Helmud is a part of El Capitan, and his heart has a pretty significant effect on El Capitan's heart. When El Capitan has the thought about being separated from Helmud, he thinks:
He wants to be that man—his own man. But there's an ache in his chest every time he thinks of it, as if Helmud's heart—which rides forever just behind El Capitan's own heart—feels the betrayal and applies sharp pressure, heart to heart. (15.6)
The love between the two is so deep that El Capitan would actually feel like he's betraying Helmud by even thinking of being his own man. Helmud is just as much a part of El Capitan as El Capitan is a part of Helmud. They define the term symbiosis — a mutually beneficial relationship between different people or groups.
But we should also recognize how unique Helmud is as his own person. It seems like he's only filled with love. He could almost be the symbol of El Capitan's heart, instead of just someone who shares the same heart. When El Capitan starts to have a mental breakdown, Helmud comes to the rescue:
Helmud wraps his arms around El Capitan, holds him tight […] Helmud keeps holding him. (43.93)
He might be the younger brother (and the one who has limited abilities), but Helmud is just as much of a man as El Capitan. And he may be a bit of a weirdo, but he more than makes up for his weirdness through his wisdom.