Every Hero has a Rarity Class attached to it. The Rarity Class affects the maximum Level und how many times the Hero can be bought. For example, there are only 20 version of each Legendary Hero.
The Different Rarity Classes
LVL Cap: 40 | Stamina Cap: 118The gray N indicated that a Hero is a Novice Hero. Novice Heroes cannot be traded with other players or sold outside the game. In other words, they only exist within the game. They are the lowest class among all rarity classes and are available in unlimited quantities. They are still very useful and can beat the highest Quest Nodes with the right Extensions.
Novice Heroes Sun Shangxiang, Blaise Pascal, and David
LVL Cap: 70 | Stamina Cap: 178
The green U stands for Uncommon Hero. They have the largest supply among all tradable Hero classes and are usually the cheapest to acquire. Their Stamina and LVL cap is also higher than Novice Heroes.
Two Uncommon Heroes, Jack the Ripper and Tokugawa Yoshinobu
LVL Cap: 80 | Stamina Cap: 208
Yellow R means Rare. As the name indicates, there's less quantity for Rare Heroes and they have more Stamina to go more frequently on Quests.
Three Rare Heroes that are being traded
LVL Cap: 90 | Stamina: 248
Epic Heroes are very rare, but are still seen relatively often. For example, Marie Curie has currently been issued 64 times.
Some Epics being traded for some larger sums of GUM
LVL Cap: 100 | Stamina: 288
Legendary Heroes are a rarely seen in this game. For example, Napoleon Bonaparte has been issued 9 times. At time of this writing, he's being listed on OpenSea at 50ETH.
Napoleon Bonaparte, one of the rarest Heroes in this game