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Donnie had always known.
It wasn’t something Splinter told him right away — not when they were young, not when they were still figuring out how to walk upright or hold training weapons. It came later, after questions started forming, after Donnie noticed things others didn’t.
Differences.
Leonardo and Raphael shared similarities, yes. Personality traits, instincts, reactions. But Mikey… Mikey mirrored Donnie in small, impossible ways. The same nervous hand movements when thinking. The same way their eyes tracked motion. Even their laughter sometimes sounded alike.
Donnie noticed patterns.
And eventually, Splinter noticed that Donnie noticed.
⸻
“You are observant, my son,” Splinter had said quietly one evening in the dojo. “There is something you deserve to know.”
That was the day Donnie learned the truth.
Four turtles had fallen into the mutagen.
But only two had shared the same clutch before that night.
Him and Michelangelo.
Biological brothers.
⸻
Donnie never told anyone.
Not Leo — their steady leader who already carried too much responsibility.
Not Raph — whose emotions burned hot and fierce.
And definitely not Mikey.
Especially not Mikey.
Because nothing would change. They were all brothers. Blood didn’t matter.
…Right?
⸻
“Donnieeee!”
Mikey burst into the lab, nearly tripping over a cable. Donnie barely caught a falling beaker before it shattered.
“Yes, Michelangelo?” Donnie sighed, though fondness softened his voice.
Mikey leaned over the workbench, grinning. “Whatcha building? Is it dangerous? Can it explode? Please say yes.”
Donnie huffed. “Ideally, no.”
Mikey laughed — bright, effortless, warm — and something tightened painfully in Donnie’s chest.
My brother.
Not just in the way they called each other family.
In a deeper, quieter way.
⸻
Later that night, during patrol, Mikey walked beside him instead of bouncing ahead with Raph or joking with Leo.
“You okay?” Mikey asked suddenly.
Donnie blinked. “Of course.”
Mikey tilted his head. “Nah. You do that thing when you’re thinking too hard. Your face gets all scrunchy.”
Donnie paused.
Of course Mikey noticed.
He always did.
“I was simply… reflecting,” Donnie said carefully.
“About science stuff?”
“…Something like that.”
Mikey nudged his shoulder. “Well, whatever it is, you don’t gotta carry it alone, y’know. That’s what brothers are for.”
Donnie’s breath caught.
Brothers.
The word felt heavier tonight.
⸻
Back in the lair, Donnie found Splinter meditating.
“You are troubled,” Splinter said without opening his eyes.
Donnie hesitated. “Sensei… do you think it is wrong that I sometimes… feel differently toward Michelangelo because of what you told me?”
Splinter finally looked at him.
“Different does not mean unequal,” he said gently. “You love all your brothers. But knowledge changes perspective. That is natural.”
“I don’t want to treat him differently.”
“And you have not,” Splinter replied. “That is why I trusted you with the truth.”
Donnie exhaled slowly.
“…Should he know?”
Splinter was silent for a long moment.
“When the time is right,” he said softly. “Family is not defined by blood alone. But neither should truth be feared.”
⸻
That night, Donnie returned to the lab to find Mikey already there, sprawled across a chair upside down.
“Couldn’t sleep,” Mikey said. “Lab vibes help.”
Donnie smiled faintly. “You say that every time.”
“Yeah, but it works.”
Mikey swung upright, studying him for a second before grinning. “You know you’re my favorite brother to hang with, right?”
Donnie froze. “You tell all of us that.”
“Yeah,” Mikey said easily. “But I mean it every time.”
He leaned back, completely at ease, trusting, safe.
Donnie felt the weight in his chest ease.
Maybe Mikey didn’t know the truth.
But somehow… he still felt it.
Connection didn’t need explanations.
Donnie adjusted his goggles and returned to work, the quiet hum of machines filling the lab while Mikey hummed softly nearby.
Brothers.
In every way that mattered.