Mycroft
In childhood, he was adored. Everything Mykie did was perfect. Sherlock wanted to be just like him. He followed him about, imitating his walk and his speech patterns. He even ordered the same food in restaurants, though he hated salmon. Everyone said that Sherlock was a miniature Mycroft.
Sometime during Mycroft's teen years, everything changed. Struggling to form his own identity, Mykie was no longer amused with his shadow. Thus began the time of rejection, cruel words and endless ridicule. Sherlock, emotions never being his area of expertise, withdrew confused and humiliated. Anger replaced adoration.
Now he thinks about his sibling as little as possible. If asked to describe him, phrases like "arch enemy" and "the world's most dangerous man" come immediately to mind. These are silly descriptors, most applicable used describing cartoon characters and superhero villains. Other words, less flamboyant and more accurate, would be: irritating, controlling, interfering, and dominating. Mycroft Holmes is an influence Sherlock would most decidedly rather do without.
It has become a game they play, attack and counterattack. There are rules to be observed, procedures to follow, insuring all is played out as carefully as the most delicate international incident. Words are weapons, honed over the intervening years to rapier sharpness. Stoic observations are met with flamboyant rebellion. It's not sentiment; it's a thing between brothers.