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.
Weaponry
The Agile wield small weapons such as 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 by just damaging then throwing them then running to a hiding spot. Also, you can just engage into conflict with them as they are really easy to counter.
Ranking
Agiles are usually found in Venice and usually hang around Brutes and more heavily armored guards.
Special Agiles
Usually, a shinier and more abstract design in armor distinguishes a higher level of skill in an agile.