Wheelchair Kathy is a Zangen employee who was mentioned in "The Champagne Reflection". She is disabled and confined to a wheelchair (hence the nickname).
Like everyone else at Zangen, Kathy was a victim of Bernadette's bullying until she had to go back to the hospital. It seems as though Kathy was the most unfortunate one in this regard, as she had to help pay for Bernadette's coffee with her secret swear jar, the bathroom that was meant for the entire floor was hogged by Bernadette (Kathy was the only one affected by this), and Bernadette was even the one who started calling her "Wheelchair Kathy".
This poor treatment towards Kathy gave Bernadette an eye opener on how rude she really is.