Talli is very very annoying, but I understand why she is that way. Gulaal and Sudha have spoiled her rotten, and the loss of her family at a very young age has made her clingy and needy. She suspects that Kesar likes someone else but is not sure, and she trusts the promise that Gulaal made to her about marrying her to Kesar. She has grown up thinking that a woman's world revolves around a man, so she is not able to focus on her exams and is instead focused on Kesar. Her only aim in life is to marry Kesar and remain in the house with respect so Jamna cannot taunt her anymore.
She has a good side when she tried to prevent Kesar from doing the wrong thing. The only annoying aspect about her is her behavior with Kesar. Its silly because that very thing turns Kesar off completely. He sees her more as a friend/little sister than girlfriend/wife material. She would not have been as clingy with Kesar had it not been for Gulaal in the first place. At this point I am more annoyed with Gulaal.
On the other hand, when Talli had to walk home and Gulaal asked her if she said anything to Kesar, like a spoilt brat she denied it and told Gulaal "you know how Kesar is" even though she kept annoying Kesar. The next day she even admitted it to Kesar. Hers is more childhood infatuation than love.