Ali Dee

Ali Dee  is the alter ego of Mona Vanderwaal. Spencer, during her stay at Radley she found out that Mona had hidden her Nurse Cap, CeCe's Visitors Pass and her own fake ID in a rocking horse after she was led their by Mona's Map and Ali.

Identity
It is unknown to the audience who exactly "Ali Dee" Is for sure. We do know that Mona Vanderwaal's face and identity are printed on an ID card to get out of Radley Sanitarium. She possibly used the name as a pun to Alison Dilaurentis's name.

Season 3
In "The Lady Killer", Mona leaves Radley dressed as nurse. Later, she is seen wearing a black hoodie and taking instructions from someone by phone. She goes to "A's" lair and reports someone, who also wears a black hoodie, that she changed plans. near the end of the episode, Mona is talking to another member of The "A-Team", who is revealed to be Toby. She tells him about her disappointment that the plans didn't work out as they should.

In "I'm Your Puppet", Spencer finds out of Mona's alter ego.