Ranks are used by the CCG to help and identify ghouls. In the game as of right now, you can only rank up by completing tasks. Here is how the ranking system works.
C-Rated: 0 Reptutation
B-Rated: 25k Reputation
A-Rated: 50k Reputation
S- -Rated: 100k Reputation
S+-Rated: 250k Reputation
SS-Rated: 750k Reputation
SSS-Rated: 1M Reputation
Rank 3 Investigator: 0 Reputation
Rank 2 Investigator: 25k Reputation
Rank 1 Investigator: 50k Reputation
First Class Investigator: 100k Reputation
Asc. Special Class Investigator: 250k Reputation
Special Class Investigator: 750k Reputation
Reputation can only be earned by completing tasks. They can be gotten at the Ghoul Cafe as a Ghoul, or at CCG headquarters as CCG.