Fear the Walking Dead is showing the beginning of the apocalypse we have come to know by the hit AMC original show, The Walking Dead. The main characters thus far are :

Travis: High school teacher romantically linked to Madison
Madison: High school counselor romantically linked to Travis
Nick : Drug addict, Madison's son, Alicia's older brother ( Senior in HS )
Alicia : High school student, Nick's sister, Madison's daughter ( Junior in HS )

They appear to be in South Central, Los Angeles. ( I used to live there and know the surrounding area pretty well).

As we were told about halfway through the episode by Tobias ( Kid who brought the knife into school ) there have been reports of this happening in 5 states, and no one has any idea what it is, except for Tobias who believes they are connected ( Which from the original show we already know ) and Nick who has seen it first hand, but is written off because he is a drug addict who must have been hallucinating. At the end of the episode, Travis and Madison witness it when they find Nick and he explains to them that he has shot his friend, but the body is now nowhere to be found. Until they start to leave and see it slowly walking in the tunnel. Travis and Madison, against what Nick is telling them, go and try to help him, until he attempts to bite Madison that is. This is the point of realization that, Nick was telling the truth the whole time and the apocalypse is among them. I believe that Travis will now take over a leadership role in order to protect his family, even though his Wife / Girlfriend Madison exhibits more leadership qualities, at least in the first episode. Alicia, Nick's younger sister sees a video about the "first" case in Los Angeles but thinks it's fake. That is, until school lets out early and her mother tells her she needs to go home and stay home for her safety.

I couldn't tell whether Nick couldn't walk normally because of the car accident, the antibiotics in his system, or because he's a drug addict. In my opinion, it's a combination of all 3. This will be interesting when they have to run away from Walkers, or when he needs some sort of hit to avoid a crash.

Also, I would love for Tobias to become a regular on the show, but we all know that won't happen.

My thoughts on the character's backstory:
I think that Madison's husband, Nick and Alicia's father died suddenly. Whether that be to a disease, car accident, could be he just got up and left, whatever the case may be, is the stem of the family. I think that Nick and Alicia used to have an inseparable bond that no one thought could ever be broken, that the absence of their father has began to ruin. I believe that this could be when Nick started using drugs, possibly as a coping mechanism for the loss of his father, which has driven him away from his family, but most importantly, his bond with his younger sister. You can tell that she is very disappointed in Nick and that she is probably taking her hurt and anger about her father out on him. You get the idea that they were really close by how she sits on the hospital bed and how their tone of voice is in their conversations.