Gas Mask Disguise

Gas Mask "A" is a member of The A-Team. This character first appeared at the end of "Now You See Me, Now You Don't." The person is seemingly revealed to be Ezra Fitz in "Grave New World" although showrunner Marlene King has more recently stated that she believes "there were two gas masks that night" and further implying that the antagonist we saw was not Ezra. His true identity remains to be fully seen.

Season 4
Now You See Me, Now You Don't

The Liars discover a photo of a World War I solider's uniform and gas mask on a board in A's Lair, next to a flier for a World War I-themed celebration of Ravenswood later that night. "A" is seen in the costume at the end of the episode, getting ready for the party.

Grave New World

Aria gets a phone call while looking for Alison DiLaurentis with the girls. The person turns out to be Ezra who is seen wearing the Gas Mask costume while spying on the girls. Gas Mask "A" is later seen following Hanna inside the old victorian mansion and locks her inside an antique phone booth in the house. Spencer later has an encounter with him in the greenhouse, and they begin to tussle. Spencer takes some gardening shears nearby and cuts his wrist. She successfully knocks him to the ground, but before Spencer is able to take off his mask, he wakes up and Spencer falls back and hits her head and gets knocked out.