Arc is stated to be Luneth's closest friends. Instead of adventuring and doing potentially fatal things such as falling down holes into unknown caves, Arc prefers to read books and learn as much as he can. However, as a result, he gets bullied a lot. Later as Arc becomes stronger, he tries to stop bullies whenever he sees them. Also, Arc is really cute.
You will notice that Arc's strength and hit points aren't as high as Luneth's (or, later, Ingus's). This is to be expected; after all, he spent his life reading and not buffing himself up. However, Arc is very fast. You should capitalize on this. Arc fits very well into the roles of Thief and later Ninja , however he picks up any job pretty well. You can even turn him into any kind of mage since his strength is rather lacking. However, if you place him within a melee role such as Dragoon and invest in it he will be just as potent as any of the other members on your team.