Gurio Umino
Umino is a boy in the same class as Usagi and Naru. He is known as the class geek; a bit of an otaku, and at first has a mild crush on Usagi.
During the course of the first season his attention shifts from Usagi to her best friend, Naru.
While Umino could be considered a comic relief character, he plays a key part in several episodes of the series, showing a courage and bravado beyond his geek-ish exterior.
He is over-exuberant about things he likes, often researching new things, and carrying around his laptop. Umino is smart, and usually along with Ami is in the top of the class, and often in the first few episodes of the first series would rub his scores in Usagi and Naru's faces.
Umino's family life is not really apparent, his mother is mentioned several times by Ikuko (Usagi's Mom) but his family is never seen. In one episode we do see the inside of his room with a giant picture of Naru above his bed.

Osaka Naru
Naru is Usagi's best friend. She's also quite often a youma battery, and a prime target for the Dark Kingdom, which starts right in the first episode when her mother's jewelry shop is attacked, and her mother replaced by the chief-youma of the day. She's a little more studious than Usagi, not a super-genius but smart enough to get by. There are hints that she works out Usagi's identity in the final episodes of the second (R) season, but if that's true she never does properly let on that she knows.
In an episode of SuperS Naru reveals that she would like to be a nurse.
Naru apparently comes from a single parent family, given her father is never shown or mentioned in the series. Over-psycho-analyzing I wonder if that is the reason she finds herself attracted to Nephrite as Sanjouin Masoto in the first season.

Pretty Guardian
In the new live action series, Osaka Naru is played by Kawabe Cheiko (spoilers through Act 15 of PGSM) who has played Sailor Mercury in Sera-Myu. But horror of horrors, she doesn't have an Umino.