The Moon Knight MCU series has made a ton of creative decisions that differ from established comic lore. However, the series has done these differences so well that many fans (new and old) are happy with them regardless. That said, it's fascinating to see how different they are.
In particular, Steven Grant could not be more different from his comic counterpart. In every conceivable way, both are the polar opposites in personality, status, and story relevance. However, the MCU's Steven Grant is arguably a lot more memorable than his comic counterpart.
Wealth
The first difference that Moon Knight comic fans will notice is Steven Grant's wealth - namely, the lack of it. He has a huge apartment as well as tons of books, but none of these are indicative of the wealth Steven Grant has in the comics. In the comics, Steven Grant is akin to Bruce Wayne.
In the comics, Steven Grant is the personality that funds all of Moon Knight's ventures. While Marc Spector had all of the artifacts he stole, his past as a mercenary meant he could not sell them safely. Thus, the Steven Grant persona handles all of that instead. In contrast, MCU's Steven Grant works in a gift shop.
Animal Lover
Steven Grant is barely a character in the comics. He is around purely to act as the reasonable businessman of the personalities and thus doesn't get a lot of focus. The MCU version of Steven Grant is fleshed out as an animal lover. One of his few friends is the one-finned goldfish sidekick Gus. Grant's love for animals also means he is a vegan.
Of course, his veganism gets challenged a lot throughout the series. For starters, going out for a steak dinner on a date and deciding to have some due to stress. Still, his love for animals is subtle but clear throughout the series.
Charisma
Steven Grant's "hat" amongst the Moon Knight personas is that of the billionaire, playboy philanthropist. In contrast, Steven in the MCU is an intelligent but socially awkward nerd. He tends to stumble over his words and is generally an introverted person who is scared of confrontation.
Steven Grant in the comics is expected to interact with the elites of society as well as do business transactions. These are not things that MCU's Steven Grant would be allowed to do. That being said, Steven's shyness is actually quite endearing, and the rare moments of confidence he has have a lot more meaning because of it.
Intelligence
In the comics, Steven Grant was by no means an idiot. After all, being a successful businessman requires a lot of cleverness and knowledge. However, that's the extent of Steven's knowledge. In the MCU, Steven Grant is an impressive academic, so much so one wonders why he's stuck in a gift shop. Steven Grant is lowkey one of the more intelligent characters in the MCU.
He fulfills the trip of the omniglot, being fluent in English and French. He's also capable of reading Macedonian and Ancient Egyptian hieroglyphics, which implies a knowledge of Coptic and Mandarin. He also has a sharp eye for riddles and puzzles, which were crucial to finding the location of Ammit's tomb. Add to that his deep appreciation of literature and Steven Grant is a wickedly cultured and highly intelligent.
British
In the mainline Moon Knight comics, all the personas are American. While Marc Spector is, as Steven puts it, the "crazy American man inside him," Steven himself is very British.
His usual dialogue is filled to the brim with British slang. Many fans find his accent endearing, but there's been debate on whether it's good or not. A possible explanation could be the fact that Steven is, after all, an identity born from Marc's idea of what a Londoner is like. So it's going to be a bit more eccentric than the average real-life British person.
Not Privy To His Alters
Steven in the comics, along with all of the other personas, is aware of the other's existence. They all work together (more or less) to be a harmonious system (again, more or less). In the series, that is not at all the case. Steven is aware that he's "sleepwalking." He restrains his ankle and has sand around his bed to know if he tried to walk out at night.
In the first episode of Moon Knight, that goes out the window when Marc, funnily enough, goes out through a window. The shifts from Steven to Marc are obviously exaggerated, but from how freaked out Steven is after these blackouts, it's obvious that he had no idea about his DID.
Hates Khonshu
Steven Grant in the comics has a pretty affable relationship with the Egyptian god Khonshu. While there are some runs where he allies with the other personalities to exile Khonshu from their mind, he actually has the least toxic relationship with their patron god compared to every other personality.
In sharp contrast, Khonshu actively calls Steven Grant an idiot and thinks very little of him in Moon Knight. He prefers his "real" champion, Marc Spector, to be on the mantle at all times. Steven also doesn't like Khonshu after Marc reveals he intends to make Layla the next Moon Knight.
Mr. Knight
Perhaps the biggest change from the comics to Steven Grant is tying him directly to the Mr. Knight persona. The Mr. Knight persona exists as an alternative to the hooded crusader look. The police view Mr. Knight as a "consultant," and therefore not the murderous vigilante they're looking for.
Mr. Knight in the comics is just as badass as the regular Moon Knight - if a bit more dapper. That is not the case (yet) in the series. In a very unique twist, the Mr. Knight persona is instead the suit worn by Steven Grant when he calls upon Khonshu's power. Considering the contrast in Marc and Steven's personalities, this is actually a very unique way to incorporate Mr. Knight into the MCU. In the future, Steven could totally be the "reasonable" consultant.