Yuna

A descendent of the great summoner Blaska. She is pure and has a whole-hearted personality. She becomes a summoner like her father in order to defeat “Sin” and embarks on a journey to gain extreme summons. She can control summon beasts. - Instruction Manual

Wakka at the wedding in Bevelle
Tidus using a sword for the first time

Overdrive: Grand Summon

Celestial Weapon: Nirvana

Author’s Note: Yuna is a character that most players end up using during their first play through but she doesn’t really add much to the party that you can’t get elsewhere. Use Rikku instead of Yuna for healing purposes.

Yuna does not have any strong attacks for most of the game (with the exception of when you obtain Holy). Her summons do come in useful during several points in the game but she does not have much else to add. Her Overdrive causes the Aeon that she summons to be able to use its Overdrive right away.

Yuna tends to be a crutch and I would use her as little as you can. She will level up enough when she is swapped into certain battles and you can use her Cure, Cura and Curaga spells outside of battle so there is no need to swap her in.