Ryuu (リユウ Ryū) was Shirayuki's previous mentor as a pharmacist. He is a 12-year-old boy and a genius pharmacist known by many in his field. When he gets absorbed in his work, he won't hear anyone calling out to him. Has an incredible fascination with poisonous plants, which made him start reading about medicinal plants in the first place.
Despite not being a part of Zen's group, he is still considered a very major character.
Ryuu has short messy brown hair and sky blue eyes. He normally wears baggy casual clothes suited for his age and a scarf. He has trousers which reach to his knee and a pair of long black socks which reach above his trousers. He also wears a pair of puffed boots.
Ryuu has a mature, calm and apathetic personality. Due to his intelligence others treat him as an adult and not a child. In addition to that he doesn't understand, or is unable to read people's feelings at times. Because of this seemingly cold personality, many people are wary of him, and a few rumors about him spread. Ryuu isn't very comfortable with everyone treating him like an adult, and it seems like he wasn't able to gain many friends due to this demeanor.
Since he was taken to the palace when he was very young, he was not able to interact with many kids that are his age. Usually he just holes himself up in his office with work, so he does not get much exercise. When he was with other children he didn’t act like one himself and didn't play with others. Because of this he had displayed his wish to 'grow up soon' a few times throughout the manga.
At a young age, Ryuu was brought to the Clarines Kingdom as he was known to be a prodigy in medicines concerned with herbs and plants. When Shirayuki passed the palace pharmacist entrance exam, Ryuu requested her to be his apprentice. In Chapter 5 Ryuu is seen with Shirayuki picking herbs and talking about why he became a pharmacist. When they return a man enters and asks for medicine. When Ryuu hands him the medicine the man drops it and says his hand has no strength, Ryuu looks at him and then tells him to get the Chief pharmacist medicine and exits the room. Moments later after Ryuu left the man talks badly about Ryuu which makes Shirayuki mad and makes the man withdraw his previous statement, which Ryuu overhears and tells Shirayuki it is not something to get heated about.
During the Lyrias arc, Ryuu accompanied Shirayuki to Lyrias as a part of the training given by the chief pharmacist, Garrack. During the widespread of the illness caused by the Olin Maris plant, Ryuu met Kirito, a boy around the same age as him, who brought a few children to the pharmacy for treatment over the illness. The two becomes close friends afterwards.
After returning from Lyrias, Ryuu and Shirayuki was sent back to Lyrias by Garrack to investigate the Olin Maris plant and see if there's a way to make the plant retain its glow whilst making it poisonless. Before, the two returned back to Lyrias, Ryuu was called by Izana for an important discussion during which Izana stated his intentions of moving Ryuu to the Wilant castle after his research on Lyrias is done.
During their second visit to Lyrias, Ryuu told Shirayuki and Obi that his joints hurts a lot lately. It was later shown that it was simply a sign that he was growing up soon which was shown around at the end of the 10 month timeskip.
Ryuu asked for Shirayuki as an apprentice because her hair color is similar to the flowers of the Yura Shigure plant. He asked her not to call him with any honorifics, even though he's technically her senior, because he thinks it's weird for an adult to call a child like that. Slowly, he opens up to Shirayuki, and is more sociable with her now. Shirayuki takes care of Ryuu, like how an older sister would.
Obi accompanies Shirayuki and Ryuu most of the time as Shirayuki's bodyguard. Because of this Ryuu also befriends him in time. Obi calls him "Half pint", "Ryuu boy" (or Ryuu-bou), and sometimes let's him ride on his back.
- "But I'm still a kid... So it's weird." —To Shirayuki (about calling his name with a honorific).
- "...I like it when it's planted somewhere that's unsuitable for it, it doesn't bloom, and instead becomes poisonous. After finding out something like that happens among plants, I started to have an interest in medicinal plants." —To Shirayuki (about the Yura Shigure plant).
- "Maybe that's because my intelligence is so uncommon that people can't treat me like a child." —To Zen.
- "Um, today... No, starting tomorrow, I have to try harder." —To Shirayuki.
- Ryuu (隆) means "noble" or "prosperous". This refers to his status as a worker in the palace.
- He works very diligently, but when his concentration is broken he automatically falls asleep in "home position" under his desk in his office. Shirayuki tried lying down under there too, wondering if it was really comfortable, and was able to sleep peacefully.
- Ryuu has a specimen of Yura Shigure on his wall, whose petals have the same apple-red color as Shirayuki's hair. Yura Shigure also appears to be his favorite kind of medicinal herb. Ryuu says it's his favorite because, "It's red and pretty".
- Ryuu seems to have low stamina and easily runs out of breath, although this may be because there is a long distance gap from the pharmacy court.
- Although he has a calm, focused and concentrated personality, once this is broken, he is easily flustered. He does not know how to deal with new occurrences.
- He ranked fourth favorite in the character poll.