"The Agent" monsters were first released in Ancient Sanctuary. At the time, they were one of a few minor archetypes. They had a few powerful effects that were generally too situational or ahead of their time for any practical use. Their boss monster was rather weak, and they lacked adequate support.
Agents can only use their "Shine Balls" once: Once they've used their "Mystical Shine Balls", there's not much else they can do except hope to draw "Kristya" or "Hyperion".
Master Hyperion: The new boss monster of "The Agents", with amazing powers. It can be Special Summoned from your hand by Banishing an "Agent" monster from your hand, field, or Graveyard, making it easy to summon. In addition to that, you can destroy a card on the field by Banishing a LIGHT Fairy-Type monster from your Graveyard. "Sanctuary in the Sky" allows you to do this twice per turn. It's also a target for "Trade-In" and has 2700 ATK, surpassing "Stardust Dragon".
Mystical Shine Ball: A Level 2 Normal Monster with mediocre stats. It's useful for Synchro Summons and making "Gachi Gachi Gantetsu" as you can Special Summon three in a turn with "Venus".
The Agent of Creation - Venus: "Venus" used to be next-to-useless, as an expenditure of 1500 Life Points to get wall monsters only meant that you'd lose more quickly. However, now it has countless uses, being able to get "Gachi Gachi Gantetsu" on its own but also able to do much more.
The Agent of Judgment - Saturn: "Saturn" has a decent effect, but only works with "the sanctuary in the sky" and if your life pointes are higher than your opponent's, usually to end the game, so it's generally not worth using.
The Agent of Mystery - Earth: "Earth" is the real reason why "Agents" are so powerful today. "Hyperion", "Venus", and even "Jupiter" contribute to making it more powerful, but "Earth" is far superior: it's one of the few Fairy-Type Tuner monsters, with the ability to search for any other "Agent" monster when it's Normal Summoned. In addition to that, while you control "Sanctuary in the Sky", you can even search for "Master Hyperion". This sets up many, many combos.
The Agent of Miracles - Jupiter: While "Venus", "Earth", and "Hyperion" were all key to making this deck work, "Jupiter" is one of those filler cards that improves consistency. In a standard deck, its 1800 ATK makes it good, but its ability go give any LIGHT Fairy-Type monster you control 800 ATK (once per turn) by Banishing an "Agent" Monster in your Graveyard makes it great - alone, it can destroy "Stardust Dragon". In addition to that, while you control "Sanctuary in the Sky", you can discard a Fairy-Type monster to Special Summon any Banished LIGHT Fairy-Type monster.
The Agent of Wisdom - Mercury: "Mercury" has a nice effect; during your opponent's next End Phase, if you have no cards in your hand, you can draw an extra card. The trouble with it is that you have to empty your hand first, which isn't very good, and that it has to be face-up, which means that Normal Summoning it (with 0 ATK) makes it vulnerable to destruction by battle.
Archlord Kristya: "Kristya" is the other reason for "Agents" being at the top of the top tiers: alone, this card can mean game over for your opponent. Not only does it stop all Special Summons, but it has no summoning conditions, and can be Special Summoned by its own effect, by returning a Fairy-Type monster from your Graveyard to your hand while you have exactly four Fairy-Type monsters in your Graveyard. In addition to that, when it's destroyed, it returns to the top of your deck, meaning that there are only a few ways to stop it. Lastly, it has 2800 ATK, equaling that of "Scrap Dragon".
Black Luster Soldier - Envoy of the Beginning: "Black Luster Soldier" is one of the cards that is shaping the format, where almost every deck attempts to use it. For "Agents", this means incorporating "Sangan" and "Tour Guide" even after the new Xyz Material rulings. It's usually well worth the space, and is often lethal when summoned.
Darklord Asmodeus: "Asmodeus" is very powerful and useful in "Valhalla" and "Fissure" builds, as it can be Special Summoned after being Banished. It also is DARK, making it easier for you to use "BLS - Envoy".
Dimensional Alchemist: "Dimensional Alchemist" is an option for "Agents", as they Banish their own monsters frequently. It's not always used, but it generally works.
Genex Ally Birdman: "Birdman" is an enabler for Synchro Summons with "Venus". By returning a "Shine Ball" to your hand you can Special Summon "Birdman", then pay 500 Life Points to Special Summon the "Shine Ball" to your hand. Then you can proceed to Synchro Summon "Trishula" or "Stardust Dragon" and Xyz Summon "Gachi Gachi Gantetsu". It also gives access to "Genex Ally Triforce".
Herald of Orange Light: "Herald of Orange Light" is an alternative to the ever popular "Effect Veiler", allowing you to negate and destroy the effect of any monster by discarding it and another Fairy-Type monster. With the +1 advantage gained from "Earth", this card's penalties usually even out to +0. It also sets up your Graveyard for "Hyperion" and "Kristya".
Honest: "Honest" is one of the most powerful monsters in the game that isn't on the Banlist: it allows a LIGHT monster to defeat any monster. It's a staple for LIGHT decks, which means that it's a staple in "Agents".
Sangan: "Sangan" is useful with "Tour Guide". Before September 1st, when it was detached you could search for a monster, but since you no longer can, it's generally just left on the field to be destroyed.
Tour Guide From the Underworld: Before September 1st, when the Xyz Rulings were changed so that Xyz Materials are no longer monsters on the field, there was a deadly combo that could be executed on the first turn: Normal Summon "Tour Guide", Special Summon "Sangan", activate "Gold Sarcophagus", Banishing "Venus", then Xyz Summon "Leviair". Use its effect by detaching "Sangan" to search for "Genex Ally Birdman" with "Sangan's" effect and Special Summon "Venus". Proceed to Synchro Summon "Trishula" or get "Gachi Gachi Gantetsu" plus "Stardust Dragon" with the possibility of "Master Hyperion" and "Black Luster Soldier - Envoy of the Beginning". After September 1st, this combo was made invalid, but "Tour Guide" still remains important if only as a way to access "Black Luster Soldier".
Gachi Gachi Gantetsu: This card is a lot more powerful than it seems at first. Not only does it power all of your monsters up, but it has to be destroyed three times to actually get rid of it. This means that your opponent must waste resources in order to kill it.
Genex Ally Triforce: "Triforce's" last effect is what makes him useful: you can Special Summon LIGHT monsters from your Graveyard each turn-this means that you can Special Summon a "Kristya" that was hit by "Solemn Warning" or "Thunder King Rai-Oh" or any other monster, including "Black Luster Soldier". The downside is that they're face-down, but you can always Flip Summon them next turn.
Trishula, Dragon of the Ice Barrier: This card Banishes your opponent's cards and summoning it often means that you've won. It's one of the most important cards in the Extra Deck.
Cards from the Sky: Basically, "Cards from the Sky" is a much weaker version of the powerful "Allure of Darkness". You have to have another monster in your hand to use it, and you lose both your Special Summons and your Battle Phase. Even with these penalties, it is still useful for the deck. Also, thanks for "Earth", you can add "Jupiter" to your hand and then Banish it for this card's effect.
Dimensional Fissure: Some "Agent" decks use this so that they can use "Miraculous Descent" more effectively.
Gold Sarcophagus: See "Tour Guide From the Underworld". Also useful in "Miraculous Descent" builds.
Pot of Duality: A semi-staple on its own, and incredbly useful in "Agents".
Sanctuary in the Sky: Alone, "Sanctuary in the Sky" contributes little to the "Agent" theme, but with them, it allows you to activate their secondary effects. Some variants use it, some variants don't.
The standard "Agent" build saw the most play at Yugioh Championship Series tournaments pre-September 1st. It fared rather well but the release of "Tour Guide" and "Reborn Tengu" made Plant Synchro practically the sole competitor for the top spot in tournaments, plus the lack of backrow destruction made it risky for the format. In the new format, this build is overshadowed by the more expensive and more powerful Chaos Agents.
"Sanctuary in the Sky" unlocks many of the more powerful "Agent" monster's effects, although it also adds redundant cards to the deck. This deck benefits greatly from "Earth's" ability to search for "Hyperion" and "Jupiter's" ability to Special Summon Banished Fairy-Type monsters.
Currently, Chaos Agent decks are one of the top competitors at all YCS tournaments, usually making up the majority of the top rankings. These decks rely on "Tour Guide From the Underworld" and "BLS - Envoy" for extra powerhouses.