Ruki is a small, petite girl. As the demon king, she has a tattoo on the back of her right forearm resembling a backwards question mark. When first introduced, she wears a brown tarp kept together by a large paper clip to conceal her appearance. She has two black wings and a skull pin as accessories for her long, pink hair. She wears a large white hoodie which covers nearly her entire body that has a yellow zipper and a red line running the length of the arms. She also has red boots, purple eyes, and pointed ears. She looks similar to her mother.
Being only 10 years old (in Chapter 1), Ruki is slightly childish. She is very level-headed, and is usually the one keeping the trio, which consisted of Alba, Ross, and herself, together. Sometimes she goes along with Ross' demeaning comments towards Alba. In Chapter 2 and onward, she becomes more mature and collected. Haruhara, the author, notes that she had become more responsible and reliable, although still childish.
She was the one who set the demons and monsters free in the human world by accidentally popping popcorn in the demon summoning pot, which was later discovered to be placed there by Dezember. She then embarks on a journey to send the demons back using her gate magic, where she meets up with Alba and Ross after Alba accidentally stabs her in the head with a dagger and then strips her clothes. She stays with them throughout their journey.
She can create portals from one place to another as well as teleport between the human and demon worlds. A special suit is needed for humans to travel with the portal. Without one, the human's ribs will crack. At least in Chapter 1, when traveling within a world with her portal, she can only move 50 meters.
- Ruki's real name is "Lumière", which is French for "light". However, other characters in the series still call her Ruki.
- The cracking ribs side effect is based on the author's own experience with cracking ribs.
- She doesn't like salty things, and doesn't mind sweet things.
- Her sister Lym is more mature than her, despite the fact that Lym's younger than her.
- August 27th, Ruki's birthday, is presumably the day Senyū is published on Nico Nico Douga.
- She is shown not to wear any clothes under her full-length hoodie for some unknown reason.
- "I'm not trying to destroy the world." - (To Alba)