Aeris is the 22 year old Ancient that resides in Midgar, tending to her flowers at an abandoned church.  She joins Cloud and Avalanche as soon as she meets him.  Aeris is great in the battlefield, having horrible physical stats but possessing extremely useful limit breaks and great magic power.

Limit Breaks
Lv. 1: Healing Wind-Aeris creates a wind that heals the entire party.
Seal Evil-Aeris causes all enemies to be temporarily paralyzed.

Lv. 2: Breath of Earth-Aeris cures the entire party of status effects.  Sorta like Quina's Angel Snack in FFIX.
Fury Brand-Aeris casts Fury on the entire party, dramatically raising their limit bars.  Extremely useful in the beginning of the game, especially with Tifa in your party.

Lv. 3: Planet Protector-Similar to using a Holy War in FFVIII, the whole party is made invincible temporarily.
Pulse of Life-This is REALLY good!  Aeris completes heals the entire party, raising their HP and MP to the maximum, fully reviving dead characters, and remedying any status effects.

Lv. 4: Great Gospel-Just as awesome as Pulse of Life, Aeris heals the entire party, and then makes them invincible temporarily.  To receive this, speak to the man in the cave at Junon for a challenge.  Whenever you fight battles, you'll have to make two numbers be the same.  Once this happens, return to him for a Bolt Ring.  Now, repeat the same process, and when the numbers are the same again, go back to him for a piece of Mythril.  Give the Mythril to a man between Gongaga and Gold Saucer.  Choose the smaller treasure chest for the Great Gospel.

Victory Dance
Aeris cleans off her clothes, and bows with her staff.