Companion Fidi is obtained by reaching a high level of trust with Dusa, and then gifting her Ambrosia.
Using Fidi in battle will summon Dusa to aid you. She will appear briefly and fire 70-damage petrifying shots at enemies for 30 seconds.
Fidi cannot be used outside of combat encounters or while fighting Hades. Hades will eventually grant the authority to use Fidi against him.
Like other Companions, Fidi can only be used once per combat encounter, and up to 5 times per run (upgradable with Ambrosia) or more if the Night Spindle is bought from the Well of Charon.
'...When Fidi first emerged from the darkness, she grew very concerned as to why she was forced to slither bodily along the ground, while shades could float about, and bats could fly. Why was she forced to move in such a thoroughly ignoble way? It was unfair. However, Mother Night implored her not to be so vain about her sense of justice; surely there were creatures who envied her. But Fidi was unconvinced. She crawled deep into a hole, ashamed, and angered by the world's cruelties. Various creatures gathered, concerned for her, and attempted to draw her out. But Fidi only dug deeper; she thought they were deceiving her. By the time Mother Night found her, she was very hungry and alone. Now, Fidi has a new perspective on the world. And she no longer needs to crawl, for she travels wherever whomever she is given to wishes to go. '
Once Zagreus forges a bond with Dusa, the text becomes "From Dusa, friend for all eternity"
It's revealed in a conversation between Zagreus and Dusa that Nyx gifted Fidi to Dusa as a reward for sprucing up the lounge (Dusa's Favor), with Dusa suspecting that Nyx knows it would be passed on to Zagreus regardless.
During a certain one-time story event, Dusa will leave the House of Hades, and will not return for several runs. Throughout this event, Fidi can still be equipped, but the associated Summon will not work in any circumstance; trying to summon Dusa may also trigger a line from Zagreus commenting on her departure.