I would try two methods:
-Create an array of all character actors or enemy actors you want the computer to choose from. Have the computer randomly select an actor from the list or choose an actor based on specific criteria.
-Create a data table of all characters to be selected in UI, and populate the character selection screen by reading through the data table. When the player selects a character, have the computer look through the data table and find a character randomly or one with specific stats