I think one of the boys who went missing from the orphanage, "Park Minwoo" (they didn't show us his picture), is the current Baek Saeon.
Grandpa Baek tried to kill him by drowning because he found out that the real Baek Saeon killed the other children, but the guy who owns the fish shop rescued him.
Ji Sangwu might be one of the twins. He also found out that the real Baek Saeon killed the children, and Grandpa Baek tried to drown him. That's what the current Baek Saeon saw. (Ji Sangwu also remembered the design on the ring)
Park Do Jae might be Ji Sangwu's twin, and he might think that Ji Sangwu is dead and that the real Saeun killed him, just like the other children. However, Park Do Jae doesn’t know that the current Baek Saeun is not the real Saeun. That's why he's taking revenge, because he thinks that Saeun killed his twin brother.
On the other hand, Ji Sangwu is trying to find his twin through the project he's currently working on. Meanwhile, the psychopath (the real Baek Saeun) is working with Park Do Jae because both of their goals are to hurt the current Baek Saeun.