Relia is the main character of Star Ocean: Integrity and Faithlessness. While she is a party member and participates in battles, she is not playable.
Found by Fidel and Miki after crash landing in a Kronos shuttle, Relia was initially very quiet and unwilling to talk. Within time, she opened up to Fidel, Miki, and the rest of the party. She is a kind and selfless girl, but is very troubled due to her circumstances. While she wishes more than anything for everybody to be happy, she realizes that for now, keeping her friends happy is most important.
Almost immediately after being found by Fidel and Miki, Relia is shown to have extraordinary powers. What was originally thought to be powerful signeturgy, Relia is imbued with Symbols by Kronos scientists as part of a series of experiments designed to create a weapon against the growing power of the Pangalactic Federation. Relia and her twin sister Feria were the only successful test subjects from these experiments.
Soon after joining the group, Relia grows attached to everyone, especially Miki, and considers them all her closest friends.
While Relia does participate in battles, she is not a playable character. During fights she will move around and block when attacked, but will not attack enemies. Instead, she casts beneficial signeturgy such as Enshelter and Power Up to boost the combat abilities of her friends. This may be due to the fact that Relia is in and out of combat too often to be useful during the main story or, perhaps, she is a child and entirely untrained in combat and Symbology.
Relia, like everyone else, can equip up to 4 roles as well as armor and 2 accessories. This opens up the ability to use roles or items that would normally be a waste on your main fighters, such as increasing SP% or EXP% without having to sacrifice combat ability.
It is also worth noting that Relia is still targeted by enemies just as any other party member is.