My theory for SKZFLIX
Felix was a guardian angel for Minho which is why he was the only one able to see Felix. Minho was driving when he got in a car crash which is why he had the bruises. Felix protected him through the crash and had a goal which was to get Minho to pursue his acting career. After it was complete, he had to leave, maybe another universe. Jeongin and Chan were able to see Felix because Felix might go to protect them next. Felix stayed with Minho for so long so he can watch him succeed. When Jeongin and Chan said that they didn't know who Felix was even though it was clear that he talked to them, it was probably because the rest of the members couldn't see him and they wanted to pass by as normal. Felix says that he will 'have to go' which could only imply him leaving Minho to go to maybe another parallel universe. When Minho and Felix meet for the last time, you can clearly see that Minho has no more bruises but Felix does, almost in the same spot the Minho did. Felix blew out a candle to make a wish, and it was for Minho to be successful and to take him pain onto him.