Canon point: Just before the start of the series. Age/Birthday: 1968, 40 years old. Species: Human. Sexuality: You will never know. Personality: Despite the hostility between them, Mycroft cares greatly for his brother, although his ways of watching out for his brother are unorthodox to the extreme.
Mycroft is shown to be much more 'normal' than his brother, and whereas Sherlock is ostracised for his behaviour, Mycroft is highly respected by his peers. However, he is seemingly just as cold and arrogant as his brother, once telling him that caring was not an advantage.
Like his younger brother, Mycroft shows an incredible talent for deductive reasoning. Perhaps due to his age and experience, it is often implied that his analytical capabilities surpass even his brother's. However, his current lack of drive and aversion to physical labour ultimately means that he usually resorts to employing his talents for security and counter-intelligence work only.
History: Sherlock's born when Mycroft is seven years old, but the now older brother falls nearly seamlessly into his new role, one that he's going to take to the extreme in years to come. Unlike his parents - who are far too keen on home schooling their two sons, he recognizes his younger brother's genius early on and tries to keep both their minds busy by challenging him with a wide array of subjects. And it isn't ever enough - not for him, not for Sherlock, but Mycroft needs something more, something that books can't give him, something his parents and brother can't give him either- And when he's eighteen, he defies the will of his parents to enlist into the British Army. Meanwhile, he feeds the then eleven year old Sherlock a lie that he's 'going abroad to study'. Which, in Mycroft's opinion isn't as much a lie as it really is. One that Sherlock's not going to believe for long, either.
But he gets home often enough and showers Sherlock with more books and other curiosa.
First he's a cadet and trudges through initial officer training, becomes a second lieutenant upon commissioning and finally becomes a lieutenant and because of his family name and talents, is shuffled into the Special Forces just in time for the Bosnian war to break out. He goes through a year of terror in Bosnia, but when he gets word of his little brother's suicide attempt, he's quick to pull the necessary strings and is by his brother's side in less than a day. Their parents are useless when it comes to dealing with their sons, so Mycroft takes responsibility for his younger brother - mainly because he feels at fault, suspecting that Sherlock has been sent to Harrow because of his impudence.
He never leaves Sherlock's side, slowly picks up all the pieces and puts his little brother back together and convinces him to go back to school. Behind the scenes, he threatens Sherlock's bullies with a few choice words and his ever growing influence in the British government, promising them in no uncertain terms that if they harm just a single hair on his brother's head, he will fuck.them.up.
Mycroft stays home based, waiting for signs of trouble that never come. After two weeks of nail biting and sitting by the phone, he leaves for Bosnia again. Three years later, the war's over and Sherlock's in Oxford, creating a future as a chemist and he's got the bests of hopes for his little brother, yes yes - and everything appears to go smoothly. Mycroft is eventually shuffled straight into the Secret Service and he falls in love there - fast and hard. He goes through the motions there, mission after mission and keeps an eye out on Sherlock from wherever he is, because nobody around his little brother know the warning signs as well as he does. And then the person he loves is killed and he keeps the ring on his finger as a reminder.
As a result of his life experiences, Mycroft grows colder, more arrogant and eventually dismisses emotions as a whole. He also quits the secret service, instead choosing to lend out his talents to the CIA and Secret Service on a freelance basis and gains an aversion to hard labour and action. His reach grows extensively in a short amount of time, enough so that he can actually be considered to be the British Government at times.
But none of that matters when Sherlock overdoses again. Again Mycroft's at his side in mere hours, eventually takes him home to their parents and even buys him a baby horse to raise to keep him busy. He eventually approaches Lestrade and convinces him to look after Sherlock, give him more things to do for when Sherlock moves back to London - and also seizes control of his brother's finances so that he can't go on a drugs bender.