Mercenaries are the rogues of V2. They can conjure magic and also have great fighting skills. They are not crippled in terms of magic like the Templars, and they aren't restricted to only casting spells like the Harakims. Mercenaries are a cross of both. Not too bad in magic, and not too bad in fighting. They would never beat a Templar in a straight up fight, and they could never out spell a Harakim. Using both fields of combat is how they succeed in the world.
Mercenaries max out all their skills at level 60. If they reach 60 strength and agility, they can choose to become Warriors. If they reach 60 willpower and intuition, they can become Sorcerers. They must defeat the Gatekeeper like all other races to become Seyan'Dus.