He is ice. That is what people see in him. No emotions, no attachments.
He pretends not to care. He even pretends to hate. And just like everything else he does, he does it perfectly. He is not a genius for nothing after all.
He noticed the only thing he ever did imperfectly when he realized that ice can be as easily broken as hearts. It only takes one single gun, pointed at a man. One single bullet, breaking his cranial bone, shattering his vaults. Destroying his archives, his memories.
"Oh, Sherlock what have you done?"
He pretends to be disappointed. He did so many times in his youth.
"I am the smart one."
He acted as if Sherlock was stupid. Told him that his puny mind was nothing compared to his big brother's genius. He managed to instill feelings of inferiority into his own flesh and blood.
"It would break my heart."
He did not lie when he uttered those words. His mind had not been addled by chemicals either. For once in his life he spoke the absolute truth. No withheld details, no hidden meanings.
When Sherlock died to the world, it did not affect him in the least. Because it did not surprise him. He had executed that brilliant plan after all. Created it, even.
It is very difficult to surprise Mycroft Holmes. The only person that ever managed to do so was his own brother the exact moment he pulled that trigger. Mycroft had felt utter astonishment and numbing shock. And for a tiny moment he had even been truly disappointed. Badly done, Sherlock. Badly done.
He was surprised by Sherlock's actions, but even more so by his own reaction.
He cared. Fondness is not a feeling Mycroft regulary cultivates.
He was frightened for his brother's safety. Before that day he had often been honour bound to pull his brother's head out of the not always proverbial sling. That is what he wanted himself to believe at least. Despite the opinion he encouraged to settle in Sherlock's mind he had not enjoyed seeing his brother being beaten up. He had been happy to see him at Christmas.
But ice simply does not do happy and that is why he hid it all behind a perfectly manifactured wall and why he never spoke a single word about it.
Because he is entierly too fond of his brother.
People deemed him to be indestructible. But he is not. In the end Mycroft Holmes is still not made of steel, but merely flimsy frost.