Rikku is an Al Bhed that happens to meet Tidus at the beginning, then joins with the party later on in the main story. She is quite energetic and upbeat, keeping a positive outlook on situations. Rikku is very loyal to Yuna, using the nickname of “Yunie,” although she is not on the side of her pilgrimage to defeat Sin.
Rikku is definitely the most agile of the party members, meaning she will get a lot of turns during normal combat. She is a cross between a thief and a chemist, as she gains access to both Steal and Use right away on her section of the Sphere Grid. She is able to pilfer items from enemies, then with Use, repurpose them for battle. Her Overdrive, Mix, is also extremely useful and probably the most powerful one during the main story, due to its utility. Despite all this, though, she lacks in certain areas, like defenses and magic. She is also one of three characters who can fight underwater.
Rikku’s section of the Sphere Grid contains a lot of Agility, as well as a good bit of HP and some Strength. She learns a number of abilities, such as Use, Steal, Mug, Spare Change and Copycat. She also has Full-Life in her section and at the end is Bribe, an ability that will be extremely useful in obtaining certain items needed for customizing endgame armor. Rikku’s section will lead her into Lulu’s, which might not be the best option, unless you want to make her a Black Mage. She should go into Auron’s section or Tidus’, to help boost her strength more.