pancakestarr:
Detective Do died last season so he won't be back. One theory is it's possible that Detective Cho and his sister Lisa are also children of Dong Bang Heon Yup. In episode 13, we learned that the said Sonang Village leader pretends to be a deity and rapes the women who believes in their false deity, One of his offspring is Dong Bang Min. Now, when Derek's mum died, she have this whirl tattoo on her wrist, indicating she used to live in the Sonang Village and since Derek and Lisa didn't know who their biological father is, it is possible that they are also a product from Dong Bang Heon Yup's molestation to their mother.
She got that tattoo after she was killed. It was the old detective's first case (the one Derek is aggressive towards). The old detective, young at the time as it was his first case, told his superior that he thinks it is a murder because the tattoo is fresh but his superior forces him to close the case as suicide. Later on, we find out that Derek's mom saw the murder of the teacher (the one that killer liked i think) and that is why she was murdered.
That being said, Derek could really be the village leader's illegitimate son. When Derek's mom said she found a way to be rich, she could have meant the village leader's scam and when the old detective meant fresh, it could be a day old when the wound still hasn't healed