This stat can be increased through Equipment that have the proper Runes, innate Skills such as the Rouge's "Dexterity", and specific Trinkets.
However, one thing to note is that in regard to Speed, Agility gives diminishing returns, meaning that the more Agility you have, the more additional Agility you will need for it to be just as effective.
The table below shows amount of Speed added based on Value of Agility:
x = 15+(agility-15)/(1+(agility-15)*0.025)
round(x+x*speed_bonus)/2 (speed bonus is the % displayed in party.)
For a quick calc
|Amount of Agility||Resulting Speed Increase|
See bottom of speed page for highest speed.
Here is the extension of the table above. Note its separated since these can't be gained with base stats and they follow another pattern for speed per agility.
Specific to Rouges, Agility also increases Damage.
The table below shows amount of Damage added based on Value of Agility:
|Amount of Agility||Resulting Damage Increase|
This means that every point of Agility increases Damage by around 0.5 Points (estimate of 0.54 for more accurate result). At base stats a rogue will have 10 str and 12 agility resulting in 13 damage. 7 damage is from str with 6 coming from agility.