If they knew, they wouldn't understand.

Well, Chad might. The large teen often understands Ichigo better then he does himself. And he has no doubt that Chad would support him. But they rest...

No, maybe he's wrong. It's not that they wouldn't understand. It's that they wouldn't approve. Renji would probably yell at him. Uryuu would lecture him. Or maybe just make his glasses glint at him. Urahara would have something to say, no doubt and probably be the one guess exactly what was going on. Orihime would give him a worried look and keep asking if he was sure he was doing the right thing.

He knows he is. He knows.

Rukia would lose her shit and try to beat some sense into him. of that he has no doubt and it's all the more reason to make sure she doesn't find out. He maybe a war hero with staggering abilities, but when faced with the petite shinigami none of that matters. He smiles, thinking how she could reach him even in his darkest times. So why was it that he'd turned to another?

"Hey! Stop daydreaming, Strawberry," Grimmjow bellows at him as he fires another cero toward the teen. The hollowfied youth bats it aside easily with Zangetsu and realizes that it only happened because his sparring partner had held onto it a few seconds extra before throwing it.

This is why it's not Rukia. Rukia does know him and there is a bond between them, but at the end of the day, she can't be what he needs.

Grimmjow on the other hand...

The Espada launches himself at Ichigo, his hands curled to bring the claws of the resurrección form to bare. He's going for the throat, snarling as he closes the distance. His hollow self begins to laugh in that eerie voice. It likes Grimmjow. It likes him a lot. It likes the fact that he can draw blood and the arrancar will do the same, that it's a struggle for dominance every time them meet.

Ichigo likes it too. Grimmjow is honest with him. Nothing is hidden. He will call him on his stupidity, never biting his tongue, never second guessing what it is that Ichigo needs to hear; what he can handle. Somehow he just always knows. That is why he's in Huaco Mundo, sparring with his one time enemy.

There is a flurry of motion as the Espada tries to get around the Ichigo's blade. Finally he bounces back and lets out three ceros in quick succession. None of them are more then half strength, given how long they've been fighting, but they don't have to be. They force Ichigo down in the air, disorient him enough that Grimmjow can get behind him, grabbing his sword arm and binding it to his chest. At the same time, sharp teeth bite down on the junction of Ichigo's neck and shoulder. He goes limp, letting the arrancar support him as they head down toward the sand dunes.

A bite to the neck is the agreed upon signal. The first to accomplish that is the winner of the fight. They put it in place because otherwise they'd be too banged up for different kinds of fun. The kind of fun his friends really wouldn't understand at all.