I think seungho must have known the person whos name jihwa used, and I guess something must have happened between them.. idk tho
From Mr Kim's reaction, we can infer that Lord Song is someone to be feared. Mr Kim seemed very worried that Seungho went to see the so called Lord by himself....
However when he got the letter, we saw Seungho laughed and didn't share Mr Kim's concerns of danger. He went right away.
That is because he recognized that it was Jihwa's handwriting. Seungho is already rilled up by Min and now Jihwa is pulling this so he went to confront Jihwa .... basically tell him to leave him alone. Remember, Seungho already warned Jihwa the first time 'not to cross the line'. By that, it must be implied that although Jihwa believes himself to have rights towards Seungho's privacy and matters, Seungho doesn't give him the access to meddle in his past or business (like Nakyum).
It's clear that Jihwa knows who Lord Song is and he had reason to believe that the letter would entice Seungho to meet him.
Seungho is mad for two reasons: he senses that Min and Jihwa are interfering with his business and he doesn't like that. He is also pissed that Jihwa used a painful past or secret to get his attention.
I understand everything so far but I’m still kinda lost on what the letter was about and why it angered seungho. Can someone ☆e l a b o r a t e☆ on it for me(⌒▽⌒) please don’t let this comment get lost because I’m so confused
EDIT: so I kinda went back to figure it out and realised that Jihwa had sent a letter to seungho pretending to be ‘Lord Song’?? Whoever that it and recognised the writing and so on. So yeah guess that’s it. Thanks for trying to helpヾ(❀╹◡╹)ﾉ~