The total number of players in the game determines how many good and evil players there are. For example, if there are six players (the minimum needed to play), four of them will be good and two will be evil.
Roughly half of all players will be "NPCs" - good players with no unique name and no special abilities. Their only job is to take part in discussions during the day, then nominate and hang players. However, in parties of 13 and above, a good NPC might be replaced by Dio, a neutral one, who is working only for himself. They also might not. In parties of 14 and above, Don Corneo is also a possible neutral NPC. It's important to note than
neutral roles win in addition to the good or evil team winning.
The other good players will have special abilities. One of them is Lucrecia, the Dreamer, who is guaranteed to be in play. Lucrecia is the strongest good player, since she can investigate whether a player is evil or not each night. How she chooses to use this information is up to her, but she should usually be careful not to reveal her role, since the evil players will want her dead. She'll also be joined by one or more randomly selected Heroes, whose abilities are a mixed bag; some of them may help a great deal, some may not help at all, and others may actually be a hindrance. Part of the game is trying to work out which ones have been included.
The default evil player is called an Executive. There will be either two or three of them at the beginning of the game, depending on the overall party size. They have no special abilities beyond knowing who the other Executives are and being able to murder people at night. In parties of 11 and above, the evil team will be assisted by Hojo, the Scientist, who can investigate a player each night to find out if they're Lucrecia. Outside of the daily public discussion, Hojo doesn't communicate with the Executives, so he also needs to be careful how he utilises his discoveries. While he is an evil player, the game ends if all Executives are killed, even if Hojo is still alive.
There are also two situational roles, which will be applied in addition to your main role. In parties of nine and above, the first person to die will become a ghost, since it can be frustrating to die before you've had a chance to do anything. Each night, they may haunt a player they have not haunted before to prevent them from speaking the next day. Haunted players can still nominate and vote for/against hangings. The ghost should follow the game closely after their death, since they still win or lose with their team. Ghosts cannot talk, nominate or vote and are forbidden from communicating with players privately, just like everyone else. Finally, in parties of 10 and above, Cait Sith will choose two people to be lovers. They will be both be told who they are in love with. In addition to their existing win conditions, lovers can win by being the last two players alive, even if they are on opposing teams. If one of them dies at any point, however, the other immediately dies of a broken heart.
A full breakdown of team balance and descriptions of all the special roles can be found
here. Note that there are two tabs.