Agile
One of the new guard archetypes is the Agile which, as the name suggests is a very fast, lightly armored enemy. They even run faster than Ezio and also have the free-run ability leading to chase sequences. The Agile were designed to flee and alert nearby guards if Ezio is successful in killing a Brute or most of the enemies rather than fight or chase Ezio and stop him if the player attempts to flee in a fight.
Combat
The Agiles fight like most enemies only with short sheathable weapons such as daggers and short blades. They mostly appear in Venice.
If you confront an agile, you can attack him until you see that he is becoming tired (you will know he is tired when he is lowering his arms and panting), then, he will be an easy prey for countering or for a combo. This is explained by the trainer in Monteriggioni.
When they do draw their weapon out, they will do just weak attacks and only attack when he gets an opportunity to strike. Since they are very fast, they will dodge most of Ezio's basic attacks. Also, Agiles are not very strong, which means they won't be able to counter Ezio's counter-kills.
Weaponry
The Agile guards are somewhat weak as they do not fight well and only do a good job of running, which explains for their small, weak weaponry and armour. The small weapons that the Agiles commonly use mostly consist of Daggers. Some of them wield Short Blades instead of daggers.
Strategy
Agiles are very fast, so taking to the roofs will not stop them. They can do all the free-run acrobatics Ezio can do. The best method of escape is jumping into water, or you can throw them into water as they cant swim (it seems Ezio is the only person in Italy able to swim). Fighting them is easy as they are easy to counter. A normal offensive move will not be very effective against an Agile because, as they are quick, they can dodge with great timing.
Ranking
Agiles are usually found in the city of Venice and usually hang around Brutes and more heavily armored guards such as Seekers.
Special Agiles
Usually, a shinier and more abstract design in armor distinguishes a higher level of skill in an agile.