Dungeon Siege 2: Broken Sword allows you to eventually add up to six controllable characters to your party, but it took me a long and arduous 45 - 60 hours to get there after starting with two.
You can control only one character at a time during combat, but you can give commands to your party members with the push of a button.
The AI included with Dungeon Siege 2: Broken Sword is intelligent. You can tell companions where to go and what to do and they'll actually complete the request.
Four character classes are included: Ranger, fighter, combat mage, and nature mage. The class of your character, however, is chosen by the path you take while playing Dungeon Siege. You can even learn special powers based on the developmental path you have chosen for your characters.
The learning curve of Dungeon Siege 2: Broken Sword is reasonable shallow, so it's pretty easy to learn to play and get right into the game without too much trouble. The game includes several challenging difficulty levels, but you have to defeat the game on easy to unlock further settings. You can play through the game using co-op with a friend via LAN or online, or you can import a party from your single player campaign.
It took me over 40 hours to make it to the veteran difficulty level and then over 40 hours to reach the elite setting of Dungeon Siege 2.