Grey's body is filled with various runes, a fair amount of which merely keep his body together, and fix it when it breaks. However, many of them... don't. These runes give Grey the ability to cast certain spells, from a fair variety of schools. They work... somewhat differently from ordinary magic. Some points:
No blood: Grey's internal runes do not require blood to cast, meaning that casting them does not physically weaken him, unless he overuses them to great extents.
Limited use: This does not mean that Grey can spam spells, for a number of reasons which we will detail.
Location: Every Internal Rune has a location, either an arm, a leg, the head, or the torso.
Heat: His runes generate heat when they are at work, and if they overheat... bad things can happen. While he could theoretically cast spells until he exploded, there are runes in place on his brain that will prevent him from casting if doing so would cause him harm. However, it won't stop it before it starts to hurt, just when it might kill him... and it is painful. Each location may only take a certain amount of heat before he cannot cast something. So, he could have a flame spell and an ice spell in his left arm, as well as a lightning spell in his right. He gets four casts of each, but he can only cast from his left arm four times: He has more different spells, but they both generate heat in the same area. It takes about an hour to go from maximum heat to 'cool'.
Now, most of his runes are 'Dormant', and awakening them is not an easily determined process. He might be in an extremely stressful situation and manage to use one by accident, or he may simply end up using one instead of something else. Either way, just happening upon it isn't enough, he must constantly train with his runes or else he won't be able to easily use them on command.
Finally, while these runes give him a wide arsenal of spells to cast, what they lack is flexibility. Let's say he has a rune that lets him toss a fireball. One might think that there's no point bringing a pyromancer along now that they have him. However, he can ONLY make that fireball. He can't change it's size, he can't control it in mid-flight, and he can never get any better with it. A true pyromancer could move the flame, shape the flame, control ambient heat, as well as improve their mastery of simpler spells. Grey can NEVER beat someone at their own blood. Additionally, his runes only come from the following schools: Vitamancy, Pyromancy, Hydromancy/Cryomancy, Electromancy, Aeromancy, Noxomancy, and at least one Arcanomancy effect (Likely something along the lines of short-ranged teleportation or force barriers).
At present, he only has one Rune available to him, Might.
Might
Location: Torso
Heat Generation: Low. Allows for around thirty seconds of continuous use.
Might is quite possibly the simplest rune he has. It merely increases his strength, temporarily endowing him with superhuman might. The trick to effectively using this rune is to know just when use it. It may not generate much heat, but one should only have it active mid-swing, when it matters. He usually turns it on for just a second, lending extreme force to a blow. While under this effect, Runes under his body glow bright purple, visible beneath his flesh...
The 42nd Gecko,
Fallenreaper,
The Irish Tree