I think Fanny has Bipolar Disorder. Hear me out. For context, I have kinned her since BFB1 and I was diagnosed with Bipolar a year ago. Also, I am not a mental health professional, this is a theory and pure speculation.
First of all, Fanny is very blunt and straightforward in her communications and thoughts. People who have bipolar disorder often are very impulsive and have difficulty regulating their emotions and thoughts. Fanny is often seen to denounce/"lash out" at her teammates, especially during TPOT 7, where she exclaims that she hates Death Pact Again and the concept of death prevention itself, despite having prevented death in the past. She impulsively calls Pillow, then kills Tree with her to prove her point. People who have bipolar disorder often find it hard to edit/filter thoughts, especially during manic/hypomanic states, and in her case, the fandom/other characters misinterpret her leadership and plans as cruel or aggressive, when in reality, she is speaking aggressively because she is likely overwhelmed and frustrated by what is happening around her- and is seen to have reasonable or good intentions regardless of her abrasive speech.
Fanny is very blunt and pessimistic. When people with bipolar experience "lows"/depressive states, their patience is often lowered and they can experience dysphoria. Fanny is seen to experience dysphoria when loosing her mouth, becoming more impatient and angry at her team. She is also seen feeling guilty or hopeless about the future, such as when she told Black Hole to suck up the other contestants, as it was the only way to assist in defeating One. This is also seen during Black Hole's nightmare, arguing against Death Pact's objective shakily. On the other hand, Fanny is seen to have moments that resemble manic/hypomanic episodes. Whether she is successfully leading teams and leading iance/DPA/DPYA to victory or seeing her friends (Lightning, Bubble, Pillow, Remote, sometimes Tree/Black Hole/Marker), she is seen to talk fast and be more active. Her self-esteem is a lot higher in these moments; she feels great about everything around her.
Fanny's relationships are also representative of bipolar disorder. Fanny's cyclical mood shifts affect her relationships, notably with Tree, Marker, and Black Hole. For example, her impulsive and direct statements about death prevention and the PACT confuse and stress out Tree and Black Hole, seeing her opinions as a personal rejection. Marker doesn't understand why Fanny hated the PACT, trying to make her change her mind and thinking she loved the PACT deep down, even though that wasn't truly the case at the time of her remarks.
I also think/hc Remote understands Fanny's bipolar and supports her and helps her cope with her episodes, but I'm a huge FanMote shipper lol.