Throughout your play of Rimworld you will come across several different types of characters. Some of these characters will be friendly such as drifters who just wander through your colony and other hostile, such as the mechanoids, looking to destroy you.
As of Alpha 14 (July 15th, 2016), traders, visitors, raiders, kidnappers, animals, etc will now escape if surrounded by walls with no way out.
As of Alpha 16 (December 20th, 2016), rescued people (especially space refugees etc) should sometimes join the colony. If the environment isn’t survivable (e.g. bad temperature, toxic fallout) they should always join the colony.
Main Article: Animals
Wild animals occasionally spawn on the map according to the biome and from random events. These wild animals and tamed animals will wander and graze on vegetation, including player-grown plants, regardless of type. Growing food outdoors can sometimes attract animals to your base perimeter. Animals are an important source of food by the meat they provide once hunted and butchered.
Raiders will target tamed animals as often as colonists.
As of Alpha 14 (July 15th, 2016), animal trainers receive 90 XP per training attempt, which is increased from 60 in Alpha 13. Taming an animal also grants 90 XP per attempt, which previously did not occur. When animals nuzzle a colonist, it counts as a social interaction and is logged in the social tab of both the colonist and animal.
Main Article: Colonists
Colonists are the living inhabitants of each RimWorld colony. Each colonist has specific skills, positive and negative traits, and a character backstory. In general, every human is a colonist except for raiders/travelers/prisoners. The colonists come in various outfits based on their background, which can be seen by looking at the 'Character' tab in game.
Main Article: Mechanoids
"Mechanoids - Autonomous intelligent robots built for domestic, industrial or military purposes. Only available to advanced cultures because such complex AI is needed to control them."—RimWorld Universe Quick Primer
Main Article: Prisoners
Prisoners are people, usually raiders, hostile tribals, or your own colonists undergoing a mental break, who have been taken captive. They are unable to leave their prison barracks and can only use facilities (such as a food dispenser or shelf) on their own if they are placed inside the prison barracks.
You should notice that your colonists won't haul food away from cells, even when directed to do so. Animals may still enter the cell on their own and eat that food however.
Main Article: Raider
Raiders, are enemies who attack in groups at random times, sometimes one after the other and others from time to time. Their group size, health, skill, and weaponry will depend on both the set game difficulty and the time already elapsed in the game. The first raid will only have one wounded raider. Subsequent groups have three to five raiders, and eventually groups can mob more than 20 with the lowest health being above 80.
Occasionally an event is triggered that spawns a group visitors from a friendly faction at the edge of the map that comes and mills about the colony. Sometimes one member of the group has items to trade, indicated by a question mark over the pawn. To initiate trade, select a colonist and right-click the trader.
A visiting group's arrival could prove advantageous if the colony is assaulted by a mutual enemy while they are present. See Events - Faction visit.
When leaving, visitors will carry away wounded members of their faction.