i think its Nandini coz she was his true love and it was for her child that he agreed to marry tanya,and you can tell that he still loves her very much, and that One of the two reasons he has changed so much (the other of course is the fact that his mum isn't around anymore) and it seems like for that reason he doesn't smile any more, has a lot of pain inside and doesn't care about anyone, but if it was tanya who had left karan before the leap instead of Nandini then, I think karan would have been much happier with or without his mother i think his position is very similar to father's. he still not over tulsi yet