Solitary Fighter is the sequel to a much better-known game, Violence Fight. Though its more of an expanded rerelease, since it has exactly the same plot, and it mostly plays in the same way, with the main (but not only) difference being the addition of a few more playable characters. This is probably a contributing factor into why it's so forgotten, but the biggest factor is definitely the circumstances of its release: it has the misfortune of being a pre-Street Fighter II-style fighting game released in 1991, the same year as Street Fighter II. So no matter what, it would have looked like a weird, awkward throwback.
But here in 2025, it's that weird awkwardness that makes it interesting: it's a fighting game, but not following the formula that SFI codified. So, it's set in 1950s America, and all of the characters are fighting for cash prizes in warehouses and other such places. Most of the stages are in a beat em up-style forced perspective arrangement, whereby you can walk in eight directions, as well as jumping. Though some stages only let you walk on a single plane (I wonder if this was an attempt at making the game look at least partially like SFII, since apparently, major changes were made to a 1992 Taito game, Dino Rex, for the same reason).
You have three buttons: punch, kick, and jump. Pressing punch and kick together makes your character crouch, and pressing either of the attack buttons together with jump does a special move of wildly varying usefulness. One of the characters, a balding fat idiot dressed in stars and stripes has headbutts instead of kicks. While you're fighting in the beat em up-style stages, sometimes armed audience members will invade the fight, but the thing is, they're just interested in violence, they don't seem to care where it goes. They'll start out attacking one fighter, then change their mind and attack the other, until they take enough hits themselves that they decide it's no longer worth the effort and give up.
Maybe the weirdest element of the game is the health bar. It goes down in inconsistent, seemingly random amounts in response to characters taking attacks, and also it'll often start going back up again immediately afterwards. And getting a character's health bar to zero isn't enough to knock them out, you've got to hit them again after that (and if it goes back up, you'll have to deplete it again). Plus, it seems like you can't win with a throw? It makes for a game that feels very imprecise and unpredictable in a way that removes any satisfaction from victory.
Solitary Fighter isn't a completely terrible game. I do really like the setting and how the game looks in general, and I am always on the lookout for games that offer takes on fighting that feel different to typical fighting games. But none of this is enough to make it good. I think the absolute best you might hope for is having one or (at most) two fights with a friend, as a little comic relief between better games.