We have been shown that Rishi have a past!For Me I think it is that Rishi is Indira's Childhood friend who is here to make him smile as she is always angry! I got this idea when i was watching the movie Mujhse Shaadi Karogi where Akshay is Salman's Best Friend on childhood and he comes to make Salman smile and happy! And start living in Salman's house! Similar is happening here where Rishi have come to stay with Sharma Family!
What You Think?