Son Myeong-hwan's gang is the unofficial name for a group of bullies led by Son Myeong-hwan. The group was no stranger to cruelty and selfishness, bullying Min Eun-Ji, Kim Cheol-soo, and Lee Jin-su mercilessly. It is unclear whether these three were the only victims of bullying or if there were more. It can be assumed they were unkind to other students besides the ones they were seen bullying, with a student saying not to wake Kim Hyeon-ju out of fear of her snapping at them.
The group is indirectly responsible for the outbreak, having bullied Jin-su to the brink of suicide, which ultimately led to Lee Byeong-chan creating the virus with the intention of it helping his son fight back, though it backfired horribly.
It can be assumed they don’t care too much for one another, with the boys not even mentioning Hyeon-ju when she was missing, instead continuing their bullying. Another example of this was when Myeong-hwan ran away, leaving Chang-hoon, who'd been bitten by Kim Kyung-mi. Even more evidence to this claim was Myeong-hwan's complete willingness to get Yoon Gwi-nam potentially killed to save himself, however this backfired when Gwi-nam killed him, having been fed up with being ordered around.