can't read Korean (raws from BishieHaven) so unable to summarise in details but here's what I gathered from the pixs.
Last chap of vol 3 - Ookil brought her back to the school to do first aid and sent her home. During the train ride, they kissed and became a couple (while the sis waited for Ookil at home and finally revealed her full face which was similar to kyung Hee's but with a scar).
News of them starting a relationship seemed to be trumpeted all over school by her friend. The story then went back in time to Ookil's childhood and how he met this other kid (christened as 'K') he admired. Ookil and Kyung Hee went out for their first date together. Ookil wanted to get her a new pair of heels from the same store (her previous pair was damaged in the scuffle with that long-hair-with-disgusting-teeth gang leader guy. Shall call him GL) but it seems the store didn't have her size. It was raining shortly after they exited the store and they shouted love-nothings to each other in the rain (cute!) before parting. Ookil then ran (back or somewhere?) to get the shoes so as to surprise her the next morning in school. Ookil then got into a fight with someone over Kyung Hee's (? not very sure it's hers or someone's else) nametag and was towed away by the teachers. The volume ended with the appearance of K (apparently just transferred) having a small interaction with Kyung Hee at school.
It seemed that by order of the school, the pair was supposed to keep a distance from one another. After a conversation with a teacher, presumbly about the same matter, Kyung Hee walked out of the school only to find Ookil blocking the path/school gate in his wait for her. The couple got into an argument - Kyung Hee was keeping her distance from Ookil and he was upset - while K watched the pair from some distance away. Ookil then went to Kyung Hee's apartment and made a din; she hurried down to keep him clashing with the caretaker. The pair ended up sitting on a bench in the park to talk over their issues and had their 2nd kiss. Ookil finally realised K had transferred to their school and chased after him (not too happy abt it thg). Ookil & KH went home together and had some cute moments on the train. When Kyung Hee reached home, she was ambushed by one of Ookil's friends who wanted to tell her (presumbly) that Ookil had some ulterior motive for dating her. KH cried alone afterwards. The pair went out on another date after school to the cinema. KH seemed to have broached the matter of breaking up (?) with Ookil while watching the movie but the pair eventually made up with another kiss.
Ookil sent KH back home after the movie, both of them in a good mood. While going off, Ookil sensed a watching presence but did not locate anyone. Apparently, there was this guy watching them behind a tree. The couple went on another date after school. While eating at a burger joint, their conversation seemed to have turned to KH's experience ('assaulted?' as a kid by the music student) but she seemed hesitant to speak about it further. Meanwhile, a motorbike gang (led by K) followed the car of the terrified teacher (who lectured Ookil & KH) on the road and the scene ended with the teacher about to be bashed up by the gang. The next day, KH ran to Ookil's classroom, faced off with him and gave him a slap (I guess it was about the teacher. Not very sure at this pt if Ookil was the mastermind but I doubt it) before running off, upset. She was alone in a room before Ookil's sis let herself into the room as well and both of them had some interaction. KH gave Ookil the cold shoulder while the latter tried means and ways to win her favor back. KH decided to forgive Ookil finally and he was estatic. He then revealed a small velvet box (for a moment, think KH thought it was a ring, hah) containing a ear stud. He then persuaded KH to pierce her ear with the stud (right at school and infront of everyone else). Once done, he looked on KH's ear stud with satisfaction (presumbly marking her as his) and kissed her on the ear.
KH was buying food at the school's cafe when she saw Ookil's sis being bulled again by others. She went back to the classroom to console her (presumbly to ask her stand up for herself) and offered her a drink. This was witnessed by KH's friend and she did not react well to the scene. The bullies took Ookil's sis's toy rabbit away and perhaps spurred by KH's words, she stood up to them but got a beating instead. Ookil got the news via his phone and ran off to the infirmary to see his sis lying on the bed (resting).
Ookil's sis was standing infront of her house when K showed up with a bubble bottle and the two had a conversation. She then went into the house while K blew bubbles outside her house. Ookil and KH was having a slight lovers' tiff - KH was recounting all the men who made a move on her seemed to be the disgusting type and Ookil was cocky about being the most eligible of them all. KH was on the train alone and thinking that she seemed to have been too easy a prey for Ookil. She was unaware of the many admiring looks thrown her way by fellow male commuters who were egging each other on to approach her. K then suddenly appeared sitting beside KH and striked up a conversation with her. KH then followed K out the train, red-faced, to the disbelief of the male commuters. Ookil and KH met in the hallway where Ookil deliberately tripped a girl in front of KH to create a commotion and thus diversion before swopping in to plant a quick kiss on KH's lips (leaving her blushing). The story then turned to Ookil's sis's encounter with K as kids and her kissing K.
KH was preparing for another date with Ookil (think it was his birthday) and wanted to take the chance to show Ookil how desirable she was. She sayshayed down the walkway in a fashionable outfit with people staring admirably at her. As she walked, she envisioned Ookil blushing as he took her in but his reaction turned out to be different. They went to the restaurant and had similar jealous reactions when members of the opposite sex showed interest in them. KH then brandished a BD cake and sang a BD song to Ookil.