Everyone is wrong! I will give you the correct explanation
I have a very long explanation about the movie in tmdb forums. Sorry, I can't repost it here because of word limit.
Here is the link: https://www.themoviedb.org/movie/293670-gok-seong/discuss/590335749251413e3800a279
In the topic I go into detail all of the hints and clues that will lead you to the correct conclusion.
I'll summarize:
The girl in white = The liar/ Evil spirit.
The forest shaman = The protector/exorcist of dead bodies; until he gets killed and possessed. He doesn't tell truths or lies, he believes that the truth won't change anyone's minds.
The hired shaman, Il-gwang = The truth sayer.
Jong-soo's deputy = Possessed liar.
Summary of important events:
1) Jong-goo and deputy go to investigate first murder.
2) That night they get visit from naked possessed lady. Later that night the evil spirit attempts to possess both of them in their sleep. Jong-goo's possession fails, but deputy gets possessed. Deputy will constantly lie and misdirect from now on.
3) On the night of investigation of second family death, Jong-goo's daughter, Hyo-jin, comes to the police station to drop off clothes and on the way back she gets possessed.
4) Jong-goo meets and interacts with girl in white/evil spirit at the site of second family death. She lies about the forest shaman.
5) Hyo-jin starts showing signs of possession compelling Jong-goo to take action
6) Jong-goo, possessed deputy and a deacon go to visit forest shaman in the woods.
7) At shaman's house they discover a ritual room and a room with lots of photos/belongings.
8) On the way back to the village, possessed deputy lies and tells deacon and Jong-goo that the forest shaman was using the pictures to possess people. This is a lie. Infact, he was using them to perform protection and soul sending rituals so that the dead couldn't become possessed.
9) Hyo-jin's possession symptoms increase because the deputy recovered her shoe from forest shaman's house. Forest shaman was using the shoe to cast a protection ritual.
10) Jong-goo and deacon visit forest shaman again and interrogate him. Forest shaman says that they wouldn't believe the truth.
11) Second shaman, Il-Gwang, comes from the city. He asks Jong-goo about someone Jong-goo had met who he was not supposed to meet. Jong-goo mistakenly says the forest shaman; infact the person he was not supposed to meet was the girl in white.
12) Il-Gwang casts a death hex on forest shaman. Forest shaman is simultaneously casting a final passage ritual on the dead guy in the car, Park Chun-Bae. Both these rituals are interrupted. Forest Shaman does not die from the death hex and Park Chun-Bae's dead body is vulnerable to possession.
13) The next day, Jong-goo and gang visit forest shaman's house in the woods yet again. They chase and eventually kill forest shaman.
14) Il-Gwang sees the evil spirit, but she can't possess him. Instead, blood spurts out of his nose and he vomits. Il-Gwang attempts to escape the village, but the evil spirit won't let him leave.
15) The deacon sees the continued deaths in spite of the forest shaman's death and starts to have doubts. He visits the forest shaman to get some answers. Forest shaman is now possessed and he repeats what alive forest shaman said previously, the truth doesn't matter because nobody will believe him. Deacon vehemently disagrees, but now it's too late to ask for the real truth because forest shaman is already dead and possessed.
16) Jong-goo meets the evil spirit and gets a phone call from Il-Gwang. Il-Gwang tells the truth about the mistaken identity of the forest shaman and tells Jong-goo not to get tempted by the spirit. The evil spirit constantly lies and tries to keep Jong-goo with her. She has a dual motive for keeping him: 1) so that he doesn't trigger the flower trap that is meant for Il-Gwang (who will enter later) 2) (possibility) She needs time to possess Jong-goo and Jong-goo has to fully believe her before she can possess him.
17) Jong-goo leaves the evil spirit and triggers the flower trap. The evil spirit is upset because she couldn't accomplish either of her goals by failing to keep Jong-goo with her.
18) Il-Gwang comes to Jong-goo's house after everyone is dead. He can pass the archeway because the flower trap is dead, thanks to Jong-goo. He takes photos of the family so that he can cast soul passage ritual, so that the dead bodies don't become possessed.
I'm not a 100% sure what the trap is that the girl was referring to. I have some theories about what it might be in the original tmdb post.