There is no ultimate right answer, you could do a number of things:
Player controls them all
If you are just starting a game, have the player create more than once character at creation and play them all. If you already started the game, either have the player create new characters to play, or take some characters that are already in the story and give control to the player.
Advantages: The player can really feel like he controls his team, and be able to influence the game more, by being able to control multiple characters, the player also has more input and effect on the game than by just controlling one.
Disadvantages: It is still a single player game, and most of the fun is in having opposing character ideologies and opinions, having everything go their way might be fun at first, but it will become less and less engaging.
Storyteller controls them all
If you are just starting a game, you should create a character or two as well, and participate in the process just like a player would. If you already started, you can create new characters to be allies, or just use those that already exist in the story.
Advantages: The player can interact with the allies without knowing how they will react, creating interesting points of friction in the game which increase overall enjoyment.
Disadvantages: As a storyteller, you already control much of the world, controlling the allies of the character means that much of the game will be you rolling against yourself, and the story will feel stagnated when the ally characters are talking to an NPC, again - you will be interacting with yourself, and the player will feel entirely left out of most of the game.
By mixing up the allies so that some of them are controlled by the player, and some by the storyteller, you can mitigate some of the disadvantages, while not losing any of the advantages.
By including more players in the game, you obviously remove all of the disadvantages, and are left only with advantages. However, this is probably something you have tried, which leads right into the more interesting part of this...
Get external input
Find a friend, family member, magic 8 ball, random number generator, dice, whatever kind of external input (the more complex the better) and "ask it a question" the more complex the external device the more complex a question (and answer) you can recieve, it can be as simple as "should I attack? (yes/no)" - flip a coin, to "what do you think about this?.... - ask a friend.
They don't need to be role players to give input on a situation, this solution can be used when the character in question is controlled by a player or a storyteller, and allows for more interesting dynamic based less on personal opinions and more on external input.