Like a Dragon Gaiden: The Man Who Erased His Name

Basta completar lutas no castelo da agua para conseguir o rank prata

This achievement is tied to the ranking system within the Castle complex, where you can improve on your standing there by winning fights in the Coliseum venue held in the complex as well as boosting your levels in the Akame Network (through completing Requests and Stroll n' Patrol missions, as well as hitting various statistical benchmarks). You'll actually earn the Silver rank automatically playing through the main story, where you're called upon by Akame to return to the Coliseum for a fight against Hammerhead. Beat him, and the cutscene afterwards will show Akame informing you of your newly earned Silver rank. The achievement should pop after this is done (or after winning the Hell Team Battle that you are then introduced to which is tied to the main story and therefore have to win to progress), the key is this achievement is actually unmissable.