Every Mii in Miitopia has a type of personality, whether it be Kind or Cool. Personality doesn't only reflect a Mii's attitude - it also grants them different quirks that activates randomly during battles and various events. A Mii's personality can be changed at any time through their profile.
Personalities have good quirks and not so good quirks. For example, a Stubborn character may defend themselves to take less damage or may refuse healing from a Cleric.
Below is the list of personalities that can be assigned to party members.
Traveler Miis encountered in Travelers' Hub also come with their own personality. Unlike the seven personalities mentioned above, Travelers' personalities don't affect the battle in any way. Also, what personality a Traveler has depends on their skills decided by their Mii's submitter (for example, a Mii with the Chatting skill will become a Chatty Mii in Travelers' Hub), which is done before submitting said Mii to Miitopia's Mii Central. The only things that are affected by the Travelers' personalities are the quest wording (though changing the Traveler via Mii Cast into another with a different personality still keeps the dialogue from the firstly-generated character) and Traveler-only events during explorations.