Sesha definitely has her own motive of revenge which is against man in portrait which resembles like Ritik.I think even against the Raheja's too,because when Shivanya will fall for Ritik and become protagonist, then their needs to be someone who is really strong and up for vengeance against whole Rajeja's, not just Ritik.It may be like this also...She is up for revenge against Ritik because he resembles like man in portrait and that's why she also hates whole Raheja family and wants to finish them.
When she was in devdi and spoke by looking at mysterious man in portrait,their was clearly so much pain in her eyes.It can't be for Shivanya but due to something really bad against her or even her Naag/lover.Their is surely more to her character,after all she ultimate antagonist of show.She in my POV also had not told the entire truth to Guru and Shivanya and has some hidden secrets.
The trio is just superb and doing mind blowing job👍🏼...RiVanya ...love them and their scenes are treat to watch