The brothers of the game's title are Kazuki Sanada and Kosuke Takaya - two friends who are always together and famous in their hometown of Tokyo (Fictional, of course). Naturally, the "Real Boys" have enemies. In the game, at the start of each mission, you will choose one of two friends as the main character in the mission. Another friend of a friend will play the role of a partner. Using an ad-hoc connection, you can import partner character data from other players.
The game has a lot of customization options for the game's two main characters, allowing you to walk the streets of Tokyo in the style you choose. You can customize over a thousand details for both characters.