Having recently watched through the series, Im pretty sure it aint the girlfriend. It's actually some guy who was simping over her for years while she friend zoned him and told him how much she wanted to get back with her ex and fuck Mulder. Works out in the end though.
Season 4-7 is probably the peak imo. Some of the filler episodes are Gold. >Morris Fletcher Body swap >Hollywood Episode>Post Modern Prometheus>Episode where Mulder geys sucked into 90's VR to waste some thot.
To name a few