Riiya's Nobody is composed of two different people which are actually Riiya's 2 personalities. One is Asuka and the other is Xira. They both share the same body as Riiya's Nobody but Xira constantly tries to suppress Asuka and maintain control over the body.

Riiya's First Personality Side of her Nobody:
Name: Asuka
Age: 15
Abilities: She writes short stories, which are actually segments of important events happening around her that are connected to Riiya. She has the ability to stop, slow, or change the time of that particular event involving Riiya during or after she's written it except she can't change the memories that follow(in other words, if Asuka changed how an event was supposed to take place, it would happen except not only would it not be able to be stopped but the people involved would remember that it wasn't supposed to happen the way it did and know it was changed). Some events she can't change, slow, or stop and she has to let them happen. The consequence, however of being able to change events of time is that Xira is able to gain just a little more control over their shared body each time.
Personality: Shy and sweet. Though she's quiet she has a very strong will and sense of justice when questioned. She's usually alone because that's all she can really be is by herself.

Riiya's Second Personality Side of her Nobody:
Name: Xira
Age: 15
Abilities: Like Riiya she is able to become invisible for very short periods of time, but her fighting skills are her best points. She is very good in both close and long-range fighting but unlike Riiya she has awful defense and has trouble utilizing Stop and never uses it, though it's supposed to be her best spell ability, so she tries to make up for it by not letting her enemies get close and leave her vulnerable and not able to fight back.
Weapons: Descending Rage and Celestial Wing
Description: Using the Zodiac's most powerful forms Descending Rage and Celestial Wing, she is able to use both simultaneously to make an almost inpenetrable fighting barrier and has mastered both weapons. Like Xigbar who is the Organization XIII member of time, she learns from him first how to use Stop but of course never learns to control it completely and use it effectively.
Personality: Quiet and reserved. Doesn't speak too much, but sometimes she does speak out against Xemnas or an older member of the Organization when her opinion becomes too strong. She develops a strange fondness of Axel during her time with the Organization, and in a way it kind of develops into a crush. She tries not to let it effect her too much and doesn't speak about it to anyone. A lot of the time she can't concern herself with him too much because she's trying to become detached from him and concentrates most of her thoughts on suppressing Asuka and keeping control over the body.
Story: Xira was born with Asuka in the same body after events in Chain of Memories(which I'll get into later) and when she discovers that she and Asuka are in the same body, she seeks a way to keep the Nobody body to herself and get rid of Asuka and comes upon Organization XIII.
At this time, a lot of the Organization members had already been eliminated by Sora, but whoever remained she tried to go to for help. She encounters Axel and he offers to take her to Xemnas with her problem.
Xira talks to Xemnas and he tells her that she could join the Organization to make up for the lost members and Xira gladly accepts thinking that he would fix her problem. But of course, he never does, and didn't really mean it when he said she could join the Organization, and Xira is manipulated by him into doing his bidding. Events begin to unfold when she encounters Riiya in later chapters(which unfortunately I can't tell you about because it'd spoil too much =P).