Thirteen

"'The owner of the reaper's cave, Thirteen has been acquainted with Solomon for over a thousand years, and often competes with him in games for his soul. She always names the traps she makes for fun, and you can say the naming is rather unique.'"

Thirteen is first mentioned as simply "the reaper" in Lesson 35-5. She is a powerful entity that watches over the "life candles" that represent the souls of humans, demons, and angels. Her first proper appearance is in Lesson 65-3.

Personality
As seen in the Devilgram Battle for the Bread!, Thirteen isn't the strongest singer. Asmodeus comments that she's "completely tone-deaf."

It seems that Thirteen has a dislike for angels. She refers to Raphael in Lesson 65-9 as an "arrogant, self-important angel," and in the same lesson Diavolo remarks that he was expecting asking a reaper and and angel to work together would be difficult.

Likes

 * Thirteen prefers small dogs over large ones because she thinks they're cuter, as stated in the Devilgram Battle for the Bread!.

Story
Note: this section may include spoilers

Main Story and Events
In Lesson 38, "the reaper" became angry at Beelzebub for accidentally breaking Lucifer's cursed record "Hymn of the Reaper." In retaliation, she caused Beelzebub's life candle to burn significantly faster and almost extinguish.

According to Diavolo in Lesson 65-6, reapers "don't fully 'belong' to any of the three worlds." This is why when looking to broaden the scope of RAD's exchange student program, Thirteen was chosen as one of the new exchange students.

Relationships
Note: this section may include spoilers

Solomon
Solomon has stated that Thirteen is very aggressive towards him, though this is because she once ate his cooking.