Ranks are a game mechanic introduced in Mario & Luigi: Bowser's Inside Story, and later appears in Mario & Luigi: Dream Team, Mario & Luigi: Paper Jam and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. They are bestowed upon a character who reaches an appropriate level through leveling up. Each new rank confers benefits on the recipient.
List of Ranks
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
Ranks first appeared in Mario & Luigi: Bowser's Inside Story, giving set rewards to Mario, Luigi and Bowser when they reach a new rank, ex. allowing the characters to equip more than one accessory and allowing the bros access to certain shops in Toad Town. Mario and Luigi have six ranks each, with the first five being named after items in the Mario series (Mushroom, Green Shell Fire Flower, Shine Sprite and Super Star), while Bowser only has four which have the words "boss" in them, referencing Bowser's status as a boss in various Mario games.
After the credits of the game, and when the ending screen is shown, Chakron's disembodied voice appears, commenting on the player's progress and the current rank they had before beating the game; either suggesting the player tries harder next time or congratulating them on getting the highest ranks.
Ranks return in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, with one of the ranks from the original game being replaced (Shine Rank, in favor for the Boomerang Rank from Mario & Luigi: Paper Jam), changing the order and level some ranks are reached, and adding one new rank for Bowser. Rank-Up Bonuses from the first two 3DS Mario & Luigi games also make a reappearance, replacing the set rewards from the original game.
Mario & Luigi: Dream Team
Ranks work slightly differently in Mario & Luigi: Dream Team than in Mario & Luigi: Bowser's Inside Story. Instead of set rewards, such as extra gear slots or shopping privileges, the player can choose a Rank-Up Bonus to give to Mario or Luigi upon reaching a new rank. Upon reaching Rainbow Rank, the player can choose two Rank-up Bonuses, and can shop in the gold side of Rose Broquet. This means that each Bro. has a maximum of five bonuses once the player reaches Level 40.
Mario & Luigi: Paper Jam
Ranks in Mario & Luigi: Paper Jam work similar to the one in Mario & Luigi: Dream Team, but there are seven ranks for every character. Once again, for Rainbow Rank the player can choose two Rank-up Bonuses, which gives the player the limit of seven slots for bonuses per character after having reached the final rank.