Tate Markham forged a lifelong connection with Webb and Hannah when she climbed out of her family's own vehicle and "squeezed though the glass and the steel and climbed over her own dead just to be with" (Prologue, 4) them. We also know her in the present day story as Taylor's mom, and since Taylor's quest to discover her identity makes up a huge part of the story, we'd say Tate's kind of an important character.
Having lost not only her parents, but also her younger sister, Lily, in the accident, Tate's life after the tragedy is marked by grief. Her emotions lack the unbreakable depth of Narnie's, however: "Despite her ability to enjoy most of her days," Hannah writes in the manuscript, "sometimes her despair was so great that, in a melancholy moment when she allowed herself to think of her family, she'd almost stop breathing" (2.72). Tate might keep living, but she never forgets.
Her relationship with Webb is largely what helps her survive these tough emotions, and the two quickly become romantically involved. After Webb disappears, Tate speculates that the accident not only brought them together, but that it was an act of fate of Romeo and Juliet-esque proportions when she looked at Webb in the car and instantly fell in love. "I heard his voice," Tate says in Hannah's book, "and it was like God spoke and I knew, from that moment on, that I'd be with him for the rest of my life. That's the only reason I lived. To be with that boy with that voice" (14.29).
Webb gives Tate hope even in the midst of unspeakable loss, but when Webb himself is killed, it sends Tate into a downward spiral that impacts not only the course of her life, but her unborn daughter's. With Webb, her pregnancy carried a hope for the future, but without him, life in Jellicoe was impossible and she ran away. This led her into a life of drug addiction, an endless string of relationships, and steadfast instability for her daughter.
Drug addiction and child neglect are pretty serious things and we can't blame you if you're feeling a little miffed about Tate's mothering abilities. Still, there's a lot of evidence that in spite of her prolonged grief from losing her family, and later Webb, Tate really does try to make the best decisions she can for Taylor. According to Jude:
"If there was anyone who could make Tate feel anything it was you, Taylor, but then somehow she'd slip up and go downhill fast." (24.151)
Ultimately, though, Tate's grief and inner demons become too much, and she gives up Taylor to the Jellicoe School.
Here's our point: It's easy to point fingers at Tate at the beginning of the book and call her a bad parent. Knowing her background, though, helps Taylor—as well as us as readers—understand the tragic string of events that broke her spirit.