This is hands-down one of my top 5 games of all-time. It's a hard game to nail down because it's very unique among video games but the closest comparison would be Shenmue. Unlike Shenmue however, RGG is interested in telling a complete story along with a complete game.
Mechanically, I would call it a 3D brawler in the vein of Streets of Rage with an RPG system where you unlock new moves, health and heat gauge. You play as two characters who have 4 unique move sets that range from slow but hard hitting to fast, agile light hitter to a balance of the two. It's also an open-world game in the vein of the early 2000s where you go around to restaurants, item stores, arcades with SEGA arcade games, and even dedicated mini-game buildings.
It's also a story-driven game while straight-forward is a rather excellent love letter to Japanese gangster series revolving around two main characters: Kiryu Kazuma and Goro Majima. Taking in 1988 Japan when the Yen was the strongest it ever was, the backdrop of a gaudy gilded age with '80s clothes, disco and Traditional Japanese culture is one of the most unique I've seen. But the real interesting story revolves around the side quests, or Sub-Stories, that tell more interesting and compelling narratives than 99% of modern movies and TV coming out in the last 5 years. Heck, there are entire plot lines, characters, and game mechanics revolving around the minigames of Pocket Kart, Cabaret, and Real Estate that most companies would sell as DLC that is just included in that game.
Also, the OST is one of my all-time favorites with some of my personal favorites being Trouble Shooting Star, Rocket Nuts Groove, Recieve You The Subtype, and Breaking Showcase.
This game is well worth getting at full price, but the sales price of $5 is an absolute STEAL for a game like this. And everyone who I've recommended this game to absolutely loves it. Get it.