E-Rank represents the second-lowest category of commercial heroes and are seen as mascots, rather than crimefighters. They also have the lowest levels of superpowers, (such as Fatman's bodyfat to muscle power), due to their low budgets. It is because of this reason that they, (and presumeably the F-Ranks), were taken off the order to capture Ratman.
However, even E-Rank heroes can raise their rankings, either by performing heroic acts, or if their businesses increase in popularity and profits, thereby affording them better powers and equipment. Fatman later gained a D-Ranking while Mr. Big Horn was promoted to a C-Rank.
This is a list of E Rank Heroes from the Hero Association.