Description and useEdit
An Agent is an operative skilled in deception and avoidance, but also trained in self-defense and the use of deadly force. Self-reliant and independent, agents devote themselves to personal goals, or to various patrons or causes.
The Agent is a stealth-oriented, minimal-combat class. The Agent is the only default class in Oblivion without an armor skill or Alteration to cast shield (with the exceptions of the Acrobat, who has Block to prevent being hit with full force, and Witchhunter, who has conjuration and thus can conjure armor or creatures to take incoming damage). As such, it is paramount for the Agent to avoid combat at all costs, or if combat is inevitable, to avoid being hit.
To prevent being forced into combat by quests, the Agent is skilled in Speechcraft. This skill can allow for non-violent solutions to many quests (but not all) so is one of the most important for an Agent. Speechcraft can be augmented by the Illusion spell, Charm. Mercantile is also important for the Agent, as the arrows needed for combat can be expensive, and are the only way an Agent can do damage.
Security and Sneak are also important for Agents. Security provides shortcuts through dungeons and buildings, hence avoiding combat. Sneak is great for use in conjunction with Marksman, but also allows for the Agent to bypass non-essential combat situations and to steal and pickpocket keys and quest items (as opposed to killing the person for the item). Sneak also provides the Agent with an alternative income other than adventuring in dungeons to gain loot (e.g. theft).
Combat is often unavoidable, so the Agent is usually a skilled Marksman and good at Acrobatics. Bows are very effective when trying to avoid combat, especially when combined with sneak attacks and the use of Acrobatics to avoid damage and jump to unreachable locations. The Demoralize spell effect is also useful when paired with Marksman, because demoralized targets will run in fear for the duration of the effect; it can be quite easy to hit the target with arrows as they flee or return to attack.
After all this, one may wonder how an Agent can be considered on par with the other classes. The answer lies in the last and possibly the most powerful skill of the Agent: Illusion. Aside from demoralize and charm mentioned above, Illusion gives the Calm spell to stop enemies from wanting to attack and Invisibility to stop people from being able to attack the player even if they wanted. Silence can force an otherwise powerful mage into using a melee weapon, like a dagger, which is much less deadly.
Also, the Agent is the only default class with both Illusion and Sneak, which is a very powerful combination. Sneaking can be augmented by the Chameleon spell, as well as gaining benefits from Night-Eye while hiding in dark areas, and many powerful spells are most effective when cast from the shadows. Frenzy raises the target's inclination to attack, and if they wants to attack, but can't see the player, they will attack someone else. This is a useful situation to be in when fighting multiple enemies: To Frenzy one of them and let the two take each other out, while the caster can take on the now-weakened winner with a stealth attack. Paralyze is a very powerful spell effect for obvious reasons, as is Command, as it allows for the player to take a break and let someone else do the fighting for them.