Board Thread:General Discussion/@comment-3354691-20160713191809/@comment-3354691-20160715060726

I believe that the practitioner has to mix his/her blood with the Cero to create the GRC, so it's different than just a more powerful Cero.

Please note that the following goes really deep into headcanon territory, and utilizes that text block I was talking about.

My theory is that the GRC can only be created by a being that has potent Shinigami powers and potent Hollow powers. The Visoreds can't use it because their Hollow powers aren't potent enough, and the Numeros can't use it because their Shinigami powers aren't potent enough.

The evolution of an Arrancar is all about becoming progressively more like Shinigami. Aizen created the Arrancars from Hollows, maybe he also also created Espada from Arrancars. Because Grimmjow lost his status as an Espada once he lost his tattoo, and got his status back as soon as his tattoo was restored, I speculate the the tattoo itself is an as-yet unrecognized Kidou that Aizen uses on an Arrancar that seeps into their blood, and that's why the practitioner's blood is used to create the GRC. Kidou is used by the Shinigami, so the use of Kidou imparts just enough Shinigami power to make an Arrancar similar enough to a Shinigami to use the GRC.

Ichigo is a special case because he had both Shinigami and Hollow powers since the day we was born. He was not a Shinigami that gained Hollow powers like the Visoreds, he was not a Hollow that gained Shinigami powers like the Arrancars, he was both Shinigami and Hollow at the same time. His powers had 15 years to mingle and merge inside him, making him a more perfect hybrid than either the Visoreds or the Arrancars.

That's the meat of my theory. My theory doesn't need any more details to be believable, but more ideas could be added to it.

We know that Aizen had been observing Ichigo and nurtured his growth since the day he was born. He probably did this because he knew of Ichigo's special heritage. Aizen was still performing Hollowfication experiments when Isshin and Masaki met; perhaps he could have found the time to analyze Ichigo's Reiatsu and learned how to impart the same kind of power to the Arrancars to create the Espada. In this version of the theory, Ichigo was the first being with the potential to use the GRC, the Espada were simply the first to capitalize on that potential.

If we want to make the theory even more convoluted, let's bring Ichigo's Quincy powers into the mix. The basic ability of a Quincy is the manipulation and absorption of Reishi from their surroundings. Maybe when Ulquiorra impaled Ichigo with his bare hand, Ichigo absorbed some of Ulquiorra's Espada powers through the wound.