Raina (nee King) Chiba is Nijiko's biological mother that married Mamoru some months before she gave birth to Nijiko.
Little background on her:
Raina is a southern girl from the US South that ended up getting into Harvard and wanting to study Anthropology - specifically the areas that deal with culture and folklore.
She's the second daughter of the King family, with her older sister, Lakeshia, already married with a few successful stores under her belt.
As a person, Raina is kind, but no push over. She's actually quite observant of other people's moods and intentions. So often she ends up guessing (usually correctly) what another person is feeling or thinking. She's an easy going kind of gal and is absolutely spoiled by her family but not in a bad way.
She can't stand when people are left out or left alone (usually through no fault of their own) so often she reaches out to help. But she also doesn't tolerate being taken advantage of.
She meets Mamoru through a group project for their class. Since the others ditched them, Raina suggests that her and Mamoru work on it themselves. This is how she learns he's got no where to go during the upcoming holidays and invites him to spend time with her family.
From there their relationship blossomed into something more. By late winter (end of February), she finds out she's pregnant. They talk it out and decided they wanted to keep not only the child but also get married a few months later in a small ceremony.
She ends up later meeting more of his friends that summer, Haruka and Michiru. She finds the two women very pleasant and likes them quite well.
Fast forward, it's been a year and a half they find out she's sick and it's terminal. Though she tries to make it easier on them, Raina obviously fears everything and wants nothing more to live but knows the impossibility of it.
She finds out one night while they're staying with her family about Mamoru's 'Other Life' and that he's a Prince with magic. Which shocks her but she realizes that she's always sensed he was more than he seemed.
She makes Mamoru promise that he would raise their child in both of their cultures.
After her death, Raina's spirit reforms and lives in Elysion with the Shitennou, Helios, and the Maidens.
There, she's referred to as Queen Raina, King Endymion's first wife.
She watches over her daughter and Mamoru.
Raina often appears as a ghost to King Endymion to listen about his day and their child. They converse often at night.
Later, she and Neo Queen Serenity meet when NQ Serenity goes to visit Elysion for the first (and last) time.
At this point, they talk. Resolving any issues either had (which wasn't much at all). After that they're friends, and NQSerenity promises that she'll find a way to revive Raina and The Shitennou.
Raina promises to be an ear for NQSerenity too since she bears a lot of pressure on her shoulders as well.
Raina as a ghost does have powers she didn't have when she was alive.
Living in Elysion has empowered her Starseed, becoming a Special Starseed (But not a Sailor Crystal).
In the future, whenever or if, she regains her body- she'll be a psychic human.