it has to be someone closer to Karan and Tulsi since karan said to Tulsi that leave it and don't go into deep or else she will be hurt. so it's either Savita, Harsh or Mihir.
I have big doubt that it is Mohini is because Gujral made a deal with her and she wants to Karan out of the house so she can be solo owner of virani if she can full Gautam.