Yossarian is the name of the white cat owned by Simon Lewis.
When Valentine and his men broke into their house to kidnap Simon in 2007, Yossarian hid under Simon's bed and, later, behind the heating vent. While Simon was on the phone with Clary, Yossarian scratched Simon, causing him to drop his phone. When Valentine finally showed himself, Yossarian yowled and was presumably knocked out, or silenced. When Clary and Luke finally arrived, Yossarian hissed angrily and launched itself at Luke, whom Yossarian presumably sensed was a werewolf. It ran off when Luke let it go.