who the fck was that

My money's on Taegu.

After the visitor leaves, the author deliberately shows a panel centered around Yeongwoon's bag, which was brought back to him.

Yeongwoon had left his bag in Taegu's car, after being dragged away by Seo In for the day. The bag was last with Taegu.

I thought so too - he just received confirmation that Seo-in & Yeongwoon ride bikes together.