That must be someone really close to Seju if he can go and sit by her bedside. Possibly a brother, distant relative or childhood friend.
I thought that Seju's whole family, bar her father, hated her so it was unexpected to see a "family member" check up on her. Whilst everyone was bitching about her in Ch19, he seemed to stay very quiet.
My wild theory is that if the guy is not blood-related then he must have somehow convinced Seju (whilst she's in her world of despair after the breakup) to marry him either for business-related matters/to get over Sumin. The scene at the end could be the guy telling Sumin that him and Seju are getting married and Sumin drops her bag in shock. That's my far-fetched guess and sure it'll be kicked to the kerb once translations are out.
I agree about Sumin's shoulder marks being a scar. Possibly self-harm?