Tilly Birch, more frequently known as Taffy, was hired by Mrs. Chatham to be Hannelore's personal assistant. However it was discovered this was just a ploy to iron out some details with Tilly's father for a business deal. This caused Hannelore to both sever ties with her mother, and get Tilly a job working on her father's space station.
Tilly first appeared one morning at Hannelore's door, claiming to be her new assistant. After explanations were made Tilly was given a 48 hour trial run, and evidently had no intent of leaving Hannelore's side. Despite overstepping some boundaries Tilly was shown to be a good assistant and friend. However after the two days were up, Beatrice revealed her real intentions for hiring Tilly, and easily considered replacing them. Infuriated, Hannelore marched up to her mother's office with Tilly booking the flight, and told her off. Afterwards, Tilly was given a job helping Dr. Chatham on his station. During one of her visits, Hannelore discovered Tilly and Station had started dating. 
Tilly is best described as business professional. They initially couldn't call Hannelore anything but Ma'am, and considered it wrong not to. They insisted on making Hannelore coffee while at Coffee of Doom, and once proclaimed they would turn themself into the Police on Hannelore's command. It was implied due to knowing Beatrice, that Tilly was more than prepared to help hide a dead body and evidence. Tilly was also quiet as Hannelore frequently called them Taffy because of her mother's mispronunciation of their name, and they frequently corrected her. It wasn't until Marten questioned what Tilly's actual name was that Tilly truly spoke up.
Tilly was also known to be incredibly ruthless, earning them the position on the station.
Tilly also has a love of puns on par with that of Claire's, causing the two to instantly become friends.