It's getting really confusing
Like why does woo hak have those flashback of past when ki ho is someone else
And ki ho is someone else because the person who is ki ho he remembers himself for his past friends to have contacted
It's most probably that bo gyeol is ki ho
But the other thing make me come up with a different story
The theory
Woo hak is ki ho. But he hurt his head and forgot every thing but eventually he had everything written in a diary all the moments as he always used to remember mom ha but never talk to people's and
No gyeol after the accident got his hands on the diary and read all of it which is why he used to wait for Mok ha at that place not ki ho as his glasses were gone (bo gyeol is never shown to use contact or having poor eyesight yk cause if he had wook hak could have already copied him)
Which is why when bo gyeol met mok ha he bought her shoes and at the end the song he could feel his brothers emotions as she was singing as he read ki ho's diary
While without any connection woo hak felt a good bond between him and mok ha.
And about his school friends talking to ki hko. Bo gyeol as an adult must have reached out to ki his friend ofc to get info on Mok ha for his brother introducing himself as ki ho cause he read how ki ho's father used to abuse him and incase he found out it would be bo gyeol so to take it upon himself.
And at the end it's just the hypothesis
Well but if really bo gyeol is ki ho then he and woo hak has to be either twins or nothing at all
Not the half brothers
Just step brothers