Lilah first appears in S01E05 although she is stated to be the object of Gabe's affection before then.
She is extremely intelligent (as stated in S01E03 by Ryan) and Gabe seeks tutoring from Ryan in order to woo her. Lilah's fighting abilities are about par with those of Ryans without his sword. Pun intended. She rejects Gabe constantly throughout the season, however, by S01E12, she seeks comfort from him on the night before the final battle.
In S01E08, Lilah is captured by Bob in order to draw out the others so he can kill them all. Whil she is his prisoner, he tortures her, slaps her, ties her to a tree, and makes her watch a bloody demonstration of what is to happen to her. This deeply emotionally scars her, as is seen in S01E12.
In the final battle in S01E12, Lilah is put in a coma by zombie 3. This has a very strong effect on Gabe, who becomes enraged at Bob Smith and saddened. Matthew Carter does all he can to try and save Lilah, and eventually gets her into a state that "it is likely she will awaken."