Rin Mate works as editor for author Midori Aoyama, who publishes under the pen name Blue Mountain. Tasked with ensuring Aoyama meets manuscript deadlines, Rin frequently pursues the author when she evades responsibilities. As deadlines loom, Rin adopts a strict stance, threatening escalated consequences for unfinished work. During less pressured times, she grants Aoyama more freedom and joins her in leisure pursuits like games of tag.

Rin attended high school alongside Aoyama, where both belonged to the literature club; this school is later attended by Syaro Kirima and Rize Tedeza. Rin was a scholarship student during her academic years. Professionally employed by Seironsha publishing, she edits Aoyama’s novels and contributes articles to magazines.

Beyond their professional relationship, Rin shares a close friendship with Aoyama. She typically addresses Aoyama formally as "Aoyama-sensei," shifting to the informal "Midori-chan" when slightly intoxicated. Though often frustrated by Aoyama's work avoidance, Rin deeply values their bond and cherishes their non-work time together. She exhibits genuine enthusiasm for Aoyama's creative work, particularly the "Phantom Thief Lapin" character, organizing promotional events like mascot costume autograph sessions. Rin holds special appreciation for Syaro Kirima’s appearances as Phantom Thief Lapin. Cocoa Hoto refers to her as "Rin-chan-san."

In the series' third season, Aoyama unexpectedly submits a manuscript on schedule. Rin responds by treating Aoyama to an outing focused purely on enjoyment, deliberately avoiding work discussions, marking a shift in their dynamic and strengthening their personal connection beyond editorial interactions.

Rin exhibits personal quirks including forgetfulness and self-consciousness about her poor timing. She avoids coffee due to its bitterness but makes an exception for Chino Kafū's special blend. Her name derives from Mandheling coffee, reflecting the series' theme of naming characters after beverages or foods.