Leonardo was the fourth sign.
The leader turtle never talked. He would wander around the house blankly, gazing at and through people and objects like a ghost, silent and watching. April never saw him do anything but train, long after Raphael was done punishing the punching bag for the day. She could count on one hand the number of times she witnessed him eat. He switched between the dojo and his room, the latter of which she never saw.
He possesed a certain power over his brothers, though. When they would misbehave, he could usually freeze them with an icy glare, or turn the full spectrum of his leader gaze on them. Leo was a cold, ruthless fighter, the best in the family, and he had no trouble drawing blood in their sparring matches.
Raphael picked fights with Leo constantly, but the blue masked turtle always put him in his place, violently, which made him even angrier.
As they grew older, he grew more and more distant, and April worried over him constantly.
Leo would suddenly pass out from exhaustion and hunger, or train until his muscles literally locked. His arms and legs were always heavily bandaged and the cloth would sometimes be stained with fresh blood, and he'd meditate for hours on end. Medicine would disappear in huge amounts from Donnie's lab, after which Leo would sleep for days, but no-one ever noticed or cared.
But the most disturbing thing about Leo was the way he treated his swords.
He adored his katanas like they were his first born. April would watch him clean and sharpen them with reverence, slicing open his own fingers along the razor sharp edges to make sure they were perfect and watching the blood make rose petal designs on the shimmering surface. Leo murmured gently to himself as he oiled them, handling the blades with long, sensuous strokes.
Once, as a "prank", Raphael had held Leo's katanas over the stove until the metal was tarnished and blackened, and the leather grips cracked and unraveled. It had taken all of them to stop Leonardo from breaking Raph's spine, and even then he almost succeeded.
April's room was directly beneath Leo's. At night, she'd hear him hissing and moaning, springs creaking as he whimpered above her head, and the soft slide and snikt of his blades against flesh.
The next morning he'd have scratches on his plastron, more bandages, and be walking with a limp, but with a small smile on his face.
April never had the courage to ask him the obvious question, even though she thought she knew the answer.
The first time she heard him laugh, it was in the middle of a slaughter of Purple Dragons. Blood was flying everywhere, arching in crimson spurts along the shining metal of his blades.
And he LOVED it.
He looked like a happy little kid, eyes gleaming and teeth bared in a feral grin as he cut through limbs and jugulars like salami.
When the bodies were still and silent, she saw him running his fingers through the coat of red on his weapons, and licking drops of it from around his mouth, shuddering in ecstasy.
That night, Google didn't have an answer for her. And she didn't want one.