I think he's just a random detective/police officer investigating the drug.
an undercover police officer who was name dropped in c25. as jy goes through his 'evidence drawer' under his fan letters near the end of that ch, we see his business card, which is one of three from jy's doctor shin kyungmin, jy's old manager han yeju, and this officer han dongju... wait a second, i wonder if there's a possibility that his old manager and this officer are related...??? they both have the last name han AND ju/주 in their name hmmm
Thank you so much for the reply I definitely missed that when I first read it!!
Can someone tell me who was the guy who got drugged inside the pool? I thought it was the therapist but I might be wrong