Who is he?
Sam is the protagonist of the 2019 game Death Stranding. In a world brought to its knees by an event known as the Death Stranding, which has upset the link between life and death, humanity has retreated to small, isolated cities, reliant on porters for communication and logistics. Sam is one of them; he also happens to be the son of the last President of the United States. It is through this bond that Sam is brought out of his apathetic exile and into BRIDGES, the organization aiming to reunite America, and sent out on a mission that could define the fate of a nation.
Moveset
That's a lot of explanation required to explain who a character is, but what can he do? As you might know, Death Stranding is, at its core, a game about delivering parcels, and it has very in-depth walking mechanics (seriously). When playing DS Sam must strategically plan his trips to account for what he'll need and what he can carry. In a fighting game that could translate to a mechanic where Sam must pick all the items and weapons he wants to have at his disposal, and be penalized with reduced mobility; or go light and risk lacking ammo or crucial items he might need in specific situations. There would also be benefits to him being at full capacity, like melee attacks being stronger due to all the weight he's putting behind them.
Aside from the overarching mechanic, Sam has tons of tools and weapons at his disposal to fight the All-Stars. His default melee weapon is the Strand, a rope that he can use to pull off complex grappling maneuvers. Other distinctive weapons in Death Stranding include bola guns, to trip enemies; sticky guns, which function basically like Isabelle's fishing rod; hematic grenades and guns, weapons that draw from Sam's blood, and so would drain his health when used; and the EX grenades (I don't know how these could translate to gameplay but I'm not going to not mention the sweat, piss and sh*t grenades). As for non-weapons, Sam can use ladders and climbing anchors for traversal (and maybe even PCCs to spawn structures and vehicles, but that's probably best for supers). That's without mentioning the difference between lethal and non-lethal weapons (which might translate to different types or amounts of damage), Sam's DOOMS abilities (maybe a slow-mo dodge like when BTs are encountered? And stuff like repatriation could work as supers) and the ability of using regular cargo as melee or thrown weapons. Basically this guy has a hell of an arsenal, and all of these items would have to be loaded onto Sam's back to be used.
Why would he be added?
I'd say there are plenty of reasons to add Sam. For one, the game was a pretty big hit, selling over 3 million copies (and that was before the PC release). Even though it was controversial, the people who liked it loved it, and it was nominated for Game of the Year at the Game Awards and won awards like Best Direction. He's also a creation of Hideo Kojima, who needs no introduction, and with Snake in Smash and Raiden having been in the last game he makes a lot of sense as the "Kojima rep" for this game (Sony owning him helps). I know many of you might not have played or liked his game, and for those people I can only offer the mental picture of Norman Reedus carrying a baby and throwing piss grenades at Kratos and Dante (to be clear this is a completely serious submission but one must appeal to the widest audience possible - and it would be hilarious).
Plus we don't have a single rep for strand-type games, that's just a glaring omission