Yuna is a pretty young girl with brown eyes and long, straight hair which she lets fall over her shoulders. Her hair colour appeared dark brown in former illustrations but changed to black in later volumes.
She is usually seen wearing a round collared T-shirt when not in the school uniform.
Yuna is a timid, quiet and benevolent girl who is also often shown to be quite naive. She is very soft-spoken.
Seo Yuna (Kor. 서윤아) is Shinwoo's and Ik-Han's childhood friend. It is later shown that she is also a childhood friend of Suyi. Yuna is a student at Ye Ran High School and is in the same class as Shinwoo, Ikhan and Rai. Shinwoo had a crush on her since they were kids. However, after he was rejected by Yuna, he decided to move on, whilst still remaining a close friend of Yuna. Yuna, on the other hand, is shown to have a huge crush on M-21.
One day, while returning home after school, Yuna sees a man slumped in an alley. She gets worried and goes to him while asking whether he was alright. However, this man is an infected person in search of food, and Yuna gets assaulted by him. Luckily, Shinwoo, Rai and Ikhan who were walking nearby, heard her screams and came to her rescue. Later Yuna is kidnapped by M-21 and M-24 to lure out Shinwoo to fight the Infected for their entertainment. After M-21 calls Shinwoo using Yuna's phone, Shinwoo and Ik-han arrive to rescue her.
M-21 tells Shinwoo to fight the Infected if he wants to free Yuna. And if he doesn't do as he says, M-21 threatens to kill them all. Due to these conditions, Shinwoo has no choice but to fight. At first, Shinwoo was only slightly suspicious of the Infected not being normal. However, as he fought, Shinwoo noticed that none of his normally lethal attacks have any effect on him. His opponent just continued fighting as though he couldn't feel the pain. M-21 and M-24 are quite shocked after seeing how strong Shinwoo was. But, naturally, being an ordinary human fighting an Infected, Shinwoo started losing, causing Ikhan and Yuna to worry.
Meanwhile, Rai and Frankenstein are on the way to find the kids. Upon arrival, Frankenstein releases his dark aura to catch the attention of M-21 and M-24. M-21 and M-24 went to meet them, thinking that they were weak. This provided Ikhan with the opportunity to move Yuna away from the fight. However, they are proven wrong when M-21 is instantly defeated by Frankenstein and M-24 is defeated by Rai. Rai and Frankenstein spare their lives. The children are saved upon the arrival of Rai and Frankenstein, although Shinwoo had gone through a lot of beatings and Yuna had trouble walking.
Yuna and Shinwoo are on their way to Frankenstein's house when they see a foreigner being robbed by some local delinquents. The foreigner, Takeo, is saved by Shinwoo. When he sees Yuna, Takeo confuses them for a couple on the date, embarrassing them. Yuna and Shinwoo join Takeo at the cafe and get acquainted. However, they are spotted by Shark who targets them as witnesses marked for annihilation.
Yuna and the gang are kidnapped by Shark and Hammer. They are taken to the DA-5's hideout and are held as hostages, in order to capture Regis. Shark also intended to use the kids to threaten M-21 as he knew them. However, Shark also states that the children can be killed now, as Takeo and Tao had contact with them. Shark states that the girls in Korea are very pretty. And when Yuna gained consciousness and asked Shark why he kidnapped them, he replies by saying that their voice is nice. Shinwoo then asks Shark why he had brought them there, Shark responds by kicking him and standing with a foot on Shinwoo's head. Regis wakes up to the sound of the kids screaming.
Now that everyone is awake, Shark calls M-21, causing the kids to notice him. M-21 doesn't want the DA-5 to know that he knows the kids. So, he tells them that he doesn't know the kids. This causes Yuna and the others to be visibly shocked. Shark gets angry but grins like a maniac, saying if that's so, then there is no use in keeping the kids alive. M-21 is distressed. He wants to find out the names of his comrades, but he doesn't want any of the kids to get hurt. In the end, without even thinking, M-21 asks Shark to let the kids go. They are later saved by Rai.
Noticing that Rai has been looking weaker, the children decide to make something nice for him to eat. Yuna is hanging out with Rai, Regis, Seira and Suyi. She and the other children want to make something nice for Rai to eat, since he's been looking weaker. Shinwoo and Ik-Han rush off to buy the ingredients, leaving her and Suyi to wonder why they were in such a hurry.
Yuna is at Frankenstein's house with her friends. Seira is cooking a meal for them, while Regis helps to serve the food. The children are talking about the story that was going around school earlier in the day, Suyi asks if Shinwoo and Ik-Han were responsible. Ik-Han denies this, however, Suyi disagrees. She tells them that since the kids at their school were attacked, by a gang looking for them, so they can't say they have nothing to do with it. Yuna agrees adding that some kids had their wallets and phones stolen. Shinwoo states that the guys said they'd return everything and not to worry. Suyi remembers the day in question and asks how they could cause trouble for other people, playing games at a PC room, and that when they showed up later than expected, they complained that it took forever to find the stuff they had to buy and had the girls massage their shoulders, because they were sore. Shinwoo and Ik-Han try to get Suyi to calm down and tell her it's no big deal. At this Suyi loses her temper. Shinwoo asks Rai for his thoughts, however, Rai remains silent. Shinwoo tries to defend himself saying that he hasn't really done anything wrong and that the two of them weren't even that late. Suyi yells they were two hours late. Shinwoo comments that Rai's face says that it's not such a big deal and why are they being harsh to their friends over nothing. Rai is confused by this and continues to remain silent. Suyi asks them what they're talking about and threatens them with a closed fist making the two jump.