Naga the White Serpent (白蛇のナーガ, Shiroi Hebi no Nāga?) is a fictional character in the novel, movie, OVA and manga versions of Slayers. Her Japanese voice actress is Maria Kawamura, and her English voice actress is Kelly Manison.

It is believed that Naga is really Gracia Ul Naga Seiryuun (Japanese: グレイシア＝ウル＝ナーガ＝セイルーン Gureishia Uru Nāga Seirūn), the older sister of Amelia Wil Tesla Seyruun. While occasionally referred to as Lina Inverse's sidekick and traveling partner, Naga's self-imagined rivalry made her just as likely to antagonize Lina during the course of any story. Naga's is presumably a few years older than the teenaged Lina, and thus takes great pride in her mature and excessively busty appearance. Within the anime convention community, Naga is infamous, as many conventions will mention that her costume, or similar costumes are specifically prohibited due to the revealing nature.

Source: Wikipedia