Eri was Shiki's best friend in the Realground. She was a clothes designer, and was the reason Shiki got into sewing. Right before Shiki died, Eri told her that she just wasn't made for designing. She later told a friend that she didn't mean it, she thought of it as a way to make her realize that something else was better for her, but she took it the wrong way. Due to this she thought about quitting designing clothes.
Eri looks exactly like Underground Shiki, because Shiki's entry fee was her own appearance. And since Shiki had so much jealousy torwards Eri at the time, Shiki was forced to take the appearance of Eri.