Responses
But, the messages they had?
Honestly, it seems more likely they weren't in a relationship. It would be more interesting plotwise. Plus the messages could have just been that. Messages. Maybe they were just really good friends in the past, idk.
Plus we've only seen like what, two shots of the messages? Not much to go off on. Saying "miss you~" is a little intimate but doesn't necessarily imply romance
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Yesung is probably lying about them being together in the past. He wants to start a new with him. I think when Sihoo was in the hospital after the fire broke Taehyuk probably took measures so Yesung couldn't learn about the incident that's why he came late.