She has been surrounded by mystery since she came onto the soap earlier this year.
Last week Fi baffled fans after flirting with colleague Mick Carter before calling Josh Hemmings, Lauren Branning’s boss.
Some speculated that she is actually Josh’s fiancée Imogen after she asked him if he “wanted to have some fun with her”.
But last night, Fi confused viewers further as she was seen making a phone call to a mystery person.
After telling Mick her father died, she then referred to the person on the other end of the phone as Dad.
Fans now think that Fi is Josh’s sister, and their dad is the owner of Weyland & Co, who Max Branning has been scheming with.
One fan said: “Got it all worked out (I think).. Josh and Fi are siblings, their dad is the chairman of Weyland, Max and Fi work together.”
Another added: “Is Fi the daughter of Max’s big boss who is also the boss of Loren’s admirer? He was in prison with Max…”
However, some viewers think that Fi is actually the daughter of Ted Murray, who was seen accidentally shooting Johnny Carter last night, after she rang him to tell him about Johnny’s condition.
One said: “I have a feeling Fi and max are a thing, Fi’s mum and dad are ted and joyce is it? And Josh is her fling.”
Another annoyed viewer added: “Josh is Fi’s brother. Fi is Ted’s daughter. Ted is an annoying t**t.”
So much mystery…
EastEnders continues tonight at 8pm on BBC.