At the start of the manhwa, Song Jae Gu was timid and very unsure of himself. Whenever he landed himself in trouble, he tried to find ways to get out without fighting, and would usually submit to any kind of abuse that he received. However, whenever his siblings were harmed, he would enter into an enraged state and act violently towards the persecutor(s).
However, this changed over the series as Jae Gu began learning how to fight from Queen, Lee Moon Young, and Choi Dal Dal. He became more confidant and more assertive in his social life, but he still retains some timidness.
When his mother left, Jae Gu became a mysogynist and a gynophobist. He at first couldn't stand being ordered around by Queen, calling her a MONSTER, nor by Choi Dal Dal or Lee Moon Young - though she isn't as pushy as the others. However, he gradually spent more time around them, and manages to have casual conversations with them, but whether he is interested in any of them or not is currently unknown.
Song Jae Gu is shown to love his little brother and sister very much.
He may not look like it, but he is quite fit, physically speaking, thanks to his part-time job. According to Lee Moon Young, he's also a fast learner at boxing and possesses good stamina, allowing him to go through six rounds of sparring right from the beginning of his training for the Wild's tournament preliminaries. He is also able to take her hits quite well , since he has been bullied, but possesses a fatal flaw, being that he is unwilling to hurt anyone. He consequently is unable to put his weight behind his punches, which is fatal in a violent tournament such as the Wild's Tournament. However, he has begun to hit back and has shown considerable strength.
His most noted attribute is his eye coordination. Several of his opponants have noted that he always kept his eyes open and perceives the attacks to the end. Several times, he's been able to counter due to his good hand-eye coordination.
|Spoiler alert. Proceed at your own risk|
As a result, Jae Gu had to replace his mother, and now barely sleeps more than few hours a night. At an unknown point in time, he obtained a part-time job at a car wash in order to earn cash.His father died a few years ago, and his mother abandoned her children shortly after the event. His mother's decision scarred him greatly, causing him to be diagnosed as a mysogynist/gynophobist.
He later applied to Wild High, and recieved a 3 year scholarship with the condition of continually attending the school. On his first day, he was accidentally pushed by Moon Young into Queen, spilling his coffee milk on her clothes. When Queen demanded payment to replace her clothes, he gave her only 280 won, when Queen was actually expecting 2,800,000 won.
Later in class, he came to find out that Wild High was a formerly all-girl school and that he was the only boy enrolled, much to his dismay. Between classes, he was ambushed by Dal Dal and blocked her kiss with his hand. Moon Young rescued him from Dal Dal and took him to Queen, where they both realized, to Moon Young's shock, that Jae Gu didn't know who Queen was.