Ondal is Pyeonggang's love that appeared in Chapter 12 - Ondal and Pyeonggang. Though, in the end, it is revealed that he is actually a figment of her fantasy.
During a hunting, Pyeonggang noticed a cute bunny near a forest. Wanting to catch it, she went too far away from her group. Now lost, she wandered around. Trying to find anyone, instead, she met Ondal for the first time. Afraid of his scary appearance, she fainted. Ondal then brought her back home.
At the next day, she went to back to the place where they met. However, a group of wild bunnies came attacked her. Just in time, Ondal arrived and protected her.
After that, she regularly invited Ondal to her home, even at one point, brought him to her room. It was at this time Pyeonggang shaved his hair, revealing his handsome face, which made her blushed. Unfortunately, Pyeonggang's older brother, Pyeonghae, came and rushed to kill Ondal. At that very time, Ondal's swordmanship was shown. In a flash, he snatched Pyeonghae's sword and had it ready at his neck. Realizing his potential, Pyeonggang advised him to become a swordman, so that none would bully him again. With his amazing ability, he rose through the ranks. By the time Munsu came, he was going to become a general.
With their fueling love, Pyeonggang proposed a marriage, but, her father objected, even prohibiting her from meeting him anymore, but he never told her why. Pyeonghae took advantage of the situation. He told her that he was not her brother, and that Ondal is her real brother. Not only that, he also planned to kill Ondal, so that no one would be in his way.
Left with no choice, Pyeonggang begged Munsu to save Ondal. However, when he came, there was nothing to be found there. This made him realized that Ondal was just Pyeonggang's imagination due to her sickness. He never exist.
However, he appeared for the last time, saving Pyeonggang from a bullet that nearly struck her.
Ondal is based on a real-life general of the Goguryeo in Korea.