SAMURAI WARRIORS: Spirit of Sanada

On the map in Ueda Castle there will be a shell drawn on the left, but if I remember correctly, not immediately, but after you advance a little in the plot. Thanks to the shells that you find in other locations, by killing Robbers , Swordmans , etc., you can open other characters from previous parts of the series for free access (but you will not be given achievements for this). Or you can give them in exchange for additional experience to upgrade your characters. This will be especially useful for leveling your character to level 99 to get the corresponding achievement.
After giving back 20-30 shells you will receive the achievement.