The same person following Alison m the store and the same person attacking her in the haunted house probably. But who can it be? Is it Mona? But she's dressed as Catwoman. I'm perplexed and can't find out who the stalker person really is... Thanks!
We don't know & it hasn't been answered.
Firstly its not Mona, it was a guy who attacked Ali. It was supposed to be Noel BUT
Noel then approaches Ali to apologize, but she tells him it was perfect—but then realizes he's apologizing because he didn't make it to the plot; he was stuck at the party, not because he had been too rough with her.
In the next scene they then show
Suddenly, a guy in an identical costume, brushes past Alison and mutters "bitch." Alison looks offended, and the guy takes off his mask as he walks away, revealing that he is Lucas.
The scene then reveals several other people wearing identical masks and burlap costumes. Someone a few paces away suspense fully takes off his/her mask, but the face is not shown.
So we can rule out: Noel, Lucas
It could be: Ian or any other guy Ali knew back then but also worked with the then "A".
This isn't answered in any subsequent episodes if i'm not wrong (correct me if i am) & we can only guess, never know, the show has so many loop holes, its dizzy.