Sparky was Alex's dog, living in the house with Alex and Jeff. Alex posted repeated videos to his YouTube channel about Sparky's antics. Sparky had become very upset at night repeatedly scratching the wall at the back of Alex's closet where there is access to a crawl space. She also whined and barked at that wall.
Alex received some deep scratches on his arm during the night that he initially attributed to Sparky but eventually realized that it was being done by the Rake. The Rake had been inhabiting the crawl space, and using it as access to Alex and Jeff's home. Sparky was simply protecting Alex by alerting on the wall of the closet.
In Alex's video Slappa' tha' bass(ah). Sparky got a new bed.
Sparky was killed at the hands of the Rake during, or immediately prior to the filming of One step forward, two steps back. Her dead body was used in a style that Evan likened to a "fishing lure" in the video, to attract Alex towards the shed, presumably The Rake was hoping to attack Alex.
In a post on her blog Damsel confirmed that Sparky's body has been recovered and she was buried the day after the incident in the video. After the incident, Alex made a sock puppet which he used to represent Sparky, much to Evan's dismay (as seen in May & June). However Alex says in A Vinny/HYBRID Christmas that he knows that the puppet is not Sparky. Whether he was just placating Steph with this statement or whether it is true is unknown.