Leta Lestrange was a close friend of Newt Scamander at Hogwarts. The two of them were outsiders and shared a love for magical creatures. When Leta took an experiment a bit too far and endangered the lives of other students, Newt took the blame and was expelled from Hogwarts. Newt keeps a picture of Leta on the wall of his shed, inside the case.
Leta was a member of the notorious Lestrange family (WFT).
David Yates discussed the character of Leta Lestrange in an interview with Cinema Blend magazine:
"Yes. Leta Lestrange comes into the second movie. She's quite complicated and damaged and confused and Newt is absolutely still in love with her, so she has a kind of power over him, and she, yeah, she's a kind of tragic figure, so we will see a bit more of her in the second movie."
"The relationship between Leta and Newt Scamander is a complex relationship, so I think there will be a lot of time for that to grow and develop, which will be fun.”
From the Web
The Mystery of Leta Lestrange by hpboy13 on MuggleNet