I never liked Dowun the moment he was introduced, and as much as I don't like his character, I rather he be upfront with her like this. He's not manipulative in the sense that he told her his intentions from the start. He could've pretended to like her, or spin a reason as to why his brother needed to do this, and she would've believed him without even questioning it. At least she's not getting tricked and he's not playing with her feelings.