Also, here is my theory for the killer based on one scene from Ep 7:
When Ye Bun and Jang Yeol ask about who may have gotten a rain poncho, the person at the store tells them that everyone has one because it's convenient to work in.
Then follows a series of scenes of people working in their ponchos. "Working" is the important bit. Seon Woo, the delivery men, and a bunch of people are shown actually using their ponchos, working in the rain.
The one exception?
The shaman.
He isn't shown working, just taking his poncho off its rack. What is it being used for?
I don't really understand what more Ye Bun could've done for Ji Suk in that moment. She knew the killer could've overpowered them both, so just running with her wasn't an option, especially with Ji Suk's ankle dragging her back. Leaving her there really was the best way to get help. As for why she didn't ask for a phone, I don't know. She should've. But she clearly wasn't thinking straight and felt quite paranoid, choosing to rely on Jang Yeol and Jang Yeol only for help. It's not the best decision, but I think it fits the situation and her state of mind quite well.
Just wanted to say thanks to whoever decided to keep the scenes of them cracking up, they made the jokes so much better Also, Shotaro Mamiya's piercing eyes omg
K-Drama 
Then follows a series of scenes of people working in their ponchos. "Working" is the important bit. Seon Woo, the delivery men, and a bunch of people are shown actually using their ponchos, working in the rain.
The one exception?
The shaman.
He isn't shown working, just taking his poncho off its rack. What is it being used for?
As for why she didn't ask for a phone, I don't know. She should've. But she clearly wasn't thinking straight and felt quite paranoid, choosing to rely on Jang Yeol and Jang Yeol only for help. It's not the best decision, but I think it fits the situation and her state of mind quite well.
Also, Shotaro Mamiya's piercing eyes omg