Big Darrell was made up specifically at first for the rape scene on Zoe, and was not a model of anybody, really.
I just needed a character that Zoe was unable to physically beat, who was able to overpower her and do what he wanted.
Since Zoe herself was a Townie, she was portrayed as being equal to the other Townies, well able to fend off any advances by them. Also, she was never portrayed as being the GF of Edgar or any other Townie (similar to Beatrice not being the GF of Ernest).
Big Darrell's presence comes at turbulent times for the Townies, since he murdered Edgar with no witnesses left alive but Jimmy, and takes charge of the Townies due to just brute strength alone. Then it is assumed he starts hitting on Zoe, culminating with the rape scene.
Over the Summer and Fall, Darrell leads to Townies to deeper law-breaking behavior, and sets the stage for them to later commit more serious crimes, such as the School Invasion with Jake, and the Kidnapping and subsequent rapes of Karen and Melody even later on. Big Darrell is later revealed to have been one of the Punks, who was cut loose by Sid for being too erratic and psychotic.