"널 실제로 본 적은 없는데" => I haven't seen you in person (...) "없는데" == 없다 + ~는데. ~(ㄴ/은/는)데 is a connective verb form giving a loose background (causal or contrasting) for what follows: - 날씨도 좋은데 공원에나 가자: The weather being fine, why don't we go to a park. - 비는 오는데 우산이 없다: It's raining and we don't have an umbrella. It is also often used as a sentence ending, for smoother and more nuanced sense. It makes it sound like there's something left unsaid. - 이거 너 주려고 만든 거야: This is what I made for you - a clear statement. - 이거 너 주려고 만든 건데: same, but sounds like there's more to it, whatever it might be. - 어떻게 할 거야?: What are you going to do? (a specific question - sounds like it demands an answer) - 어떻게 할 건데?: And? What do you think you'll do? (lighter, like in an ongoing conversation) - 시험날은 다가오는데: The test day is nearing (and what am I doing?). - 이래서는 안 되는데: I shouldn't be[act] like this (but I can't help it).

없다. isn't. doesn't. 널 실제로 본적은 없는데 : I haven't seen you but ...