For all the negativity Bella and Edward claim she holds, Jessica has clearly proven herself otherwise in her attempt to save Bella from her own stupidity on a couple of occasions; in which any real "jealous and spiteful" person as she is claimed to be would have left Bella to rot (as would have any sane person for that matter).
Also her later relationship with Mike Newton shows her being a character foil. While Bella leaves Mike for the perfekt Eddie and he chooses Bella over Jessica, the two left overs get together. But in comparison to the special and perfect (not really) relationship of Bedward, Jessica and Mike seem to have problems in their relationship (as in any normal relationship!). Even though they don't have the special problems of being a vampire and a human, they don't accomplish to have a perfect relationship as our protagonists. Maybe this is because Mike and Jessica aren't special enough or maybe they are just normal and sane and understand when its time to let go.
There is also Bella's actions after she and Jessica attend a movie. Because Bella needs dangerous excitement to recall Edward, she places herself AND Jessica's safety in jeopardy. Bella feels compelled to mix with the unknown motorcycle gang. Jessica manages to get Bella to safety... but Bella really isn't concerned and never thanks Jessica. On their way home in the car, Bella doesn't understand why Jessica won't speak to her anymore.