Gray Morrison was a perfect example of the dangers involved in the process of trying to become a knight or mage. Due to his innately crippled body he could not become a knight as he could not train his body. To add insult to injury his mana veins, the vessels responsible for transporting mana; the substance that fueled mages and knights , were also weak which just increased the difficulty if him being a mage so exponentially that it was the same as saying he couldn't be a mage.
This left him a depressed boy as becoming a powerful knight or mage was the dreams of everyone in this world. When his tenth birthday came and he was suppose to undergo the talent test to determine whether he had the talent to practice magic or become a knight he pestered his father to let him take the test which he agreed to out of sympathy for his son. That turned out to be a mistake he regretted because Gray was apparently gifted with potential to become a mage.
Mana in one. There is no mana for mages and mana for knights ,it's just one power source the difference only lies in the utilization. Mages use mana to cast spells, which requires exceptional understanding whilst knights use mana to fight with their physical bodies which requires a well trained body. The latter is the reason Gray could not become a knight as he's body was innately crippled meaning not even healing spells could cure him as magic requires essence to cast and take effect.
You cannot create something out of nothing. That is a magic principle that the lower levels of mages abide by. As a storm is created because there is air and water, similarly healing spells work on the basis of returning the body to it's origin through the injection of vitality. Gray's body was crippled as it's origin meaning not even high class healing spells could work on him. Perhaps a spell from a great mage could work but they were elusive beings that were rare to come by. Even in the whole of the Evergreen kingdom there was only one known great mage on the surface, who was supporting the kingdom and acting as a detterent to other kingdoms and rarely took action.
The result of his mana sensitivity was an average aptitude but he had good affinity for the ice element. This potential but inability to become a mage resulted in him becoming even more frustrated and in that frustration he thought what if he could create a miracle and become a mage despite what they all told him. He secretly bought a meditation technique from the black market after pulling together his savings and money his father had allocated for his upkeep as he travelled a lot due to him beinga merchant. When all that was done all he had to do was act. He pretended to come to terms with his condition and became a cheerful young boy and as he kept up that act all the vigilance somewhat became lax and he could stroll around the estate in his wheelchair without any constant supervision as long as he was somewhere safe in the mansion.
This gave him a golden opportunity to meditate and start his journey towards becoming a mage. Perhaps his talent and determination were really good as he was able to sense mana after only year of secret meditation without guidance but as he tried to absorb this mana perhaps due to excitement he made a slight mistake which wouldn't be an issue if he was a normal person. He's body's mana veins being inhenrently weak could not bear any slight deviation and this lead to mana rampaging inside of his body. He's screams could be heard throughout the mansion as he's body was being torn apart from the inside and without suspense he dies allowing for Brian to transmigrate into his body.