WARNING: Spoilers for Moon Knight episode 1.
It's odd that Moon Knight is giving any focus to a goldfish, yet he holds a key to the mystery of who Steven Grant is. In his London flat, Steven Grant (Oscar Isaac) affectionately cares for a one-finned goldfish named Gus, who was even on his own promotional poster released by Marvel prior to the show's release. With Moon Knight's debut episode on Disney+ even being titled «The Goldfish Problem,» there is clearly an intentional focus on Gus, however large it may be.
Gus helps exemplify Steven's seclusion in life. Steven's talks with Gus are among the few moments in the debut in which he communicates with someone who will not respond back, shown also in his voicemails to his mother and his conversation with the statuesque street performer. Looking at all of Moon Knight's characters, not many of them are really friends of Steven. Even Layla (May Calamawy) is only familiar with Marc Spector, the main personality to whom Steven is an alter. Steven is relatively alone in his world. He's aware of his mental health issues and consequential social struggles, which his interactions with people prove. Still, there's added significance to Gus considering the attention both Steven and Moon Knight provide him, even if it's mainly for one episode.
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Gus provides some metaphorical explanations about Steven. For one, goldfish are thought to have notoriously short memories. However, science has actually debunked this about goldfish, with experiments showing that goldfish remember objects for sometimes months on end. Steven seems to lack any recollection of his life as Marc and has frequent bouts of confusion throughout the episode,
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