I have 2 theoriesđ
1. Nobody gave Anu the letter, she found a copy of it and came to her own conclusions and realized Niks did love her.(because the letter was folded and we did not see it had been mended and she never said to Niks that Veer gave it to her or that anyone gave it to her). having said this it means that neither Niks or Anu are non the wiser about Veer and they have some more misery ahead of themđ
2. Anu is very clever and someone like Amma gave her the torn letter she was told about Veer but wants Veer to now Suffer but she does not want to break Niks heart as thats his bestest friend in the whole wild world (yeah...right). So she decides to play along and will be the downfall of Veer...I hope it is this as this is the Anu we LOVE!!!!đ¤I hope your back Anu, make Veer pay!!!
This means the shadow could be
1. Tulika/Laila or someone who is also out to get Anu and Niks
2. Anu herself!!!đ
3. or (to add a twist) it could be someone who saw who knows everything about how evil Veer is and wants to let him know he is here to protect Niks and Anu and that is ANUS DAD.đ