Like a Dragon Gaiden: The Man Who Erased His Name

This achievement requires you to recruit fighters that can join you in the Hell Team Rumble events held in the Coliseum. Some fighters can be found to pick up at the start from the Fighters Lounge you're introduced to in the main story, as well as when you beat them in Coliseum events, where they'll show up in the Fighters Lounge afterwards (you'll have to interact with them with for them to join, some will offer a question or challenge that requires a correct answer, but you can try again right away if you happen to get it wrong). Other fighters can join you as a result from you completing Requests in the Akame Network that they're tied into (examples being the leaders of the Red Peacock, White Knight and Black Swallowtail gangs as well as Masaharu Kaito [Yep, Yagami's partner in crime in Judgment!]) There are also wealthy clientele you can find in the Castle complex that will offer you fighters they've scouted for your hard-earned yen in exchange. They can be found in the Coliseum Lobby (sitting in one of the chairs on the left to the stairway of the VIP Lounge), VIP Lounge (seating area around the pool) and the High-Rollers area of the Casino (on one of the couches leading up to the room of Blackjack tables). You should be able to get the 20 fighters that you need for this achievement once you make note to explore new areas of the Castle and meet back with Akame at the Sotenbori hideout for new Requests as you win fights, move up your Rank, and get a nice large bank account from your winnings in the process.