Reiko is Kai's mother. She works as a prostitute and hates both the job and the men involved.
Growing up at the Edge of the Forest pretty much sealed her fate—she didn't really have other options. She's got this deep love for the sea despite having never actually seen it, which is why she named Kai after the word for "sea." It's kind of a window into how she dreams of a different life.
She was just 15 when she had Kai, making her only 25 at the start of the manga. People often note she looks way younger than her actual age.
Content compiled by AnimeList.moe from publicly available sources.
