If you have a few Stark characters out and during plot phase valar is played, Is Meera reed killed or put into shadows? Roommate was trying to argue since a stark character leaves play during valar she would be put into shadows …is this right ?

Nope, all the characters become moribund dead pile simultaneously and once a card is moribund with a specific destination it cannot have another effect try to put that card in another destination.

Said another way, since Valar kills everything, including Meera, she is dead before she can Respond to the death of the other Stark characters by jumping back into Shadows. Once she is dead, she cannot "un-die" by leaving play in a different way (unless a card effect specifically says she does something "instead" of going to the dead pile).