Rikku is an Al-Bhed, a fictional race from Spira. Rikku is the cousin of Yuna and and she also has a father named Cid and an older brother named, Brother.
In Final Fantasy X, Rikku is portrayed as the perky young girl. She is funny, cute, and loves teasing the other party members. Especially Tidus, most likely because she can sense his crush on her cousin, Yuna.