A character is an organization or individual involved in an interaction.
If a Character represents more than one person
A character added to a frame should be a group that will consult separately about whether or not to implement its stated intentions when the time comes for intentions to be implemented. For example, the US decision in 1920 not to join the League of Nations, after having campaigned for it, meant that it was a separate character. As in this case, a character will generally be made up of sub-characters (the President, Congress, etc.) that decide its preferences, beliefs, position, etc., through their internal interactions.
Other names
In ConfrontationManager a character is called a Party.