He's nothing like either of his brothers, except for a passing resemblance to the younger of them.
They're incredibly smart, and have confidence in spades; even their unusual names are a reminder of how far from ordinary they both are. While he's just little Martin, with his ginger hair and his general awkwardness around people.
He hates always being the stupid one. Hates that his brothers laugh at him whenever he says he wants to be a pilot, despite being unable to fly a model aircraft.
"Sometimes I wish I wasn't born," he tells his parents upon his first failure at getting his CPL. "Why did you bother at all?"
Mummy and Father exchange a worried look, one that doesn't escape his notice. Oh. So he's right, they've never really wanted him in the first place.
"I need some air," he announces, bolting for the door. It's Mummy who goes after him, patiently explains that they needed a donor for their youngest son that was suffering from childhood leukaemia.
"That doesn't mean we don't love you, Martin," she assures him, but he's not sure he's inclined to believe her.
He's just a saviour sibling, born not for his own sake but to save another – a brother who is constantly teasing him about his lack of mental prowess, when it comes to that.
It doesn't really matter, for he's eighteen now and he may as well start a new life somewhere else. Anywhere but here, this is all that matters.
xxx
He's using his mother's maiden name now, so that people won't link him to his self-important brothers. Captain Martin Crieff doesn't sound that great, but all things considered it's a price he's willing to pay.
Especially when one of his brothers is occupying a 'minor' position in the British government, while the other has just appointed himself as the world's only consulting detective.
When he starts working at MJN Air he carefully avoids mentioning his family; not that Carolyn would care in the slightest, but he already has to endure Douglas' relentless teasing without him knowing how superior Martin's siblings actually are.
Soon enough his colleagues at MJN become his second family, in spite of Douglas' superiority and Carolyn's bossy attitude. They're friendly enough when they want to be, while Arthur is overfriendly by nature so he's not sure he actually counts.
He still calls Mum and Dad from time to time, no matter that they always mention how dangerous his brothers' occupations are. Martin has never enjoyed the thrill of danger; all he wanted was to be a pilot, and he supposes he's one now.
He's flying an aeroplane at least, even if he doesn't actually get paid to do that.
xxx
Douglas notices he's worried, but for once his first officer is more supportive than teasing. He doesn't fully understand the game one Jim Moriarty is playing with his brother; but it seems that the consulting criminal is determined to destroy Sherlock's reputation, and Martin feels bad about that.
Still he doesn't mention it with his friends, until the news of Sherlock's suicide hit him like a bolt from the blue. His brother has jumped to his death, and he hasn't been able to save him.
He was supposed to save him, that's Martin's only thought as guilt washes over him like a tide.
Arthur finds him crying in the galley, drags him to the cabin where Carolyn shoves a cup of tea into his hands.
"Tell us what happened, Skip," the boy prompts him as he takes a careful sip of the hot beverage. "We want to help."
"I – thank you, but I'm afraid there's nothing you can do."
A moment later Douglas barges in. "Here you are, Martin. Why aren't you answering your phone?"
"I'm sorry, I can't –"
The first officer cuts his rambling short, hands him his own phone. "It's your brother. Apparently he's been trying to call you all day."
"Mycroft," he murmurs hoarsely as he struggles to hold back the tears.
"Martin. What's the utility of a mobile phone if you don't even bother to use it?"
"It's all over the news. I hardly need you to confirm it."
"That was never my intention. I need your help, Martin. Now, listen carefully to my instructions."
He blinks repeatedly, then does exactly as he's been told.
When he's finally faced with his not quite dead brother, the only thing he can do is throw his arms around him and pull him into a somewhat awkward embrace.
xxx
There's something different about the way Carolyn and Douglas look at him now. Two years ago Mycroft Holmes has required the services of MJN Air to sneak their brother out of the country, and he's doing it again now that he has to extract Sherlock from Serbia.
Quite understandably Arthur is left in the darkness about it all, for the boy couldn't lie even if his own life depended on it. But then Arthur always thinks that everybody is brilliant, Martin included, so it doesn't make that much of a difference when it comes to the newfound respect the young captain has finally earned.
Sherlock looks terrible indeed, but still manages a weak smile when he spots his little brother. A moment later his head is resting against Mycroft's shoulder, as sleep takes over him at last.
Something warm stirs inside Martin's chest, and even while he knows he'll never truly belong to his siblings' world he has to acknowledge that family bonds are stronger than any of them will ever admit.
xxx
His heart is heavy in his chest as he prepares to take off for a destination that is strictly classified according to MI6 procedures. Rationally he knows that what Sherlock has done is wrong; his brother has shot a man in cold blood, and that's a crime regardless of his actual motives.
However, he also knows how much friends actually matter. And in spite of their earlier divergences, he's not prepared to lose Sherlock anytime soon.
"You could save him, if only you wanted to," he told Mycroft upon their last meeting. "You're the smart one, aren't you?"
"That I am, brother mine. Rest assured that I'll do anything in my power to ensure our brother's survival."
They're only four minutes into their flight when the sat-comm starts ringing. A grin breaks onto Martin's face when he hears of Jim Moriarty's supposed return, and is asked to turn the plane around.
He doesn't doubt for a second that this is Mycroft's doing, and for once he's secretly grateful that his brothers are far smarter than he could ever be.