She could be the naagin in the story that Shesha gave Shivanya to read, in which she falls in love with a human. She could be that naagin and maybe that's why the last few pages were torn...
That can also be a possibility. Considering how she knows a lot about naagins. She could actually be the naagin karuki who had a tragic ending in that book of which we know little about. She's probably behind the naagmani to get her powers back hence she used Ankush and his friends as scapegoats to get her job done as she was able to evade the dying naagin from capturing her picture in her eyes as she might have already known about it happening before itself . Besides this she probably had some sort of enemity with Shivanya's family as they could have possibly responsible for her tragedy hence that incident was probably her revenge.