Basic Profile: Rikku is a young Al Bhed girl, or a race of people who go against the Yevon religion and instead use Machina, which have been outlawed. She suffers from prejudice in Spira since most of the people hate her race of people, but even so she remains optimistic and perky. She is also the first person that Tidus encounters when he ventures into Spira. She tells him the most basic information about Sin and that Zanarkand had been destroyed 1000 years beforehand, which confuses him greatly.
Tidus meets up with Rikku again in the Moonflow where she becomes a permanent member of the party.
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In-depth Information This information is spoiling, so don't read it if you don't want to be spoiled!
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Rikku's father is Cid, the leader of the Al Bhed. Her brother is Brother, and she is one of Yuna's cousins from the Al Bhed side of her heritage.
Rikku's mother died shorly after giving birth to Rikku. Rikku always wanted to have more siblings, but it was impossible since her mother had died. When younger, Rikku was in the water trying to defeat a fiend when her brother got his aim wrong with a Thunder spell and accidently hit Rikku. Ever since then, Rikku had been afraid of Thunder due to the incident, and it became apparent ingame when the party traveled through the Thunder Plains.
During the game after meeting up with her in the Moonflow, Rikku keeps talking about how she doesn't support Yuna's pilgrimage to defeat Sin. This causes a great deal of confusion for Tidus. Later on when Home, Where Rikku was from, was being destroyed, Tidus learns from the young Al Bhed the real purpose behind a summoner's pilgrimage and ultimately ends up feeling the same way as Rikku does about the whole situation.
Not wanting Yuna to die, both of them try to come up with a plan to save Yuna together.