It's a pretty simple project and, other than narrative theory, I can't think of much second hand research that could be done simply because it's looking into how characters can push the story onwards. After a few searches, I haven't found anything on the subject either, so this is mostly personal experience and observations based on games that I've played.
Story driving characters is really the best way we can describe what
we're going to be talking about. We're looking into characters that
help push the story along by guiding the player in the right
direction and giving the player purpose to their actions and
direction. Most characters do this, but there is almost always one
character that seems to have this specific purpose. They are normally
a companion that guides the player and aids them along the way by
helping to solve puzzles or helping to fight the enemy, or they can
be a character that narrates to the player and points them in the
right direction without getting directly involved. In other forms of
media, every character drives the story forwards, even just a little,
by directing and aiding the main protagonist. Everything is in a set
direction. On the other hand, games are a bit more lenient. While
story driven games do still have a story, it's not in such a set
direction as other forms of media due to the fact that the player can
explore and interact with the characters and environment. The player
can choose their own pace and experience the story in their own time
and as such the game needs characters that guide the player.
Early examples of such characters can be found in the 80's where
games were becoming more in-depth and complex and started to include
characters other than the player that wasn't enemies. Characters
started to appear that would give the character quests and items that
would compel them to carry the story on. Without them, not much would
have happened. However, they sometimes didn't have much interaction
with the player that was beyond telling them to go to the next area
or giving them equipment. More recently, as games have become more
complex, these characters can do much more. One recent example is
Elizabeth from Bioshock: Infinite. Elizabeth is the main part of the
story as her abilities and who she is are the very reason for the
existance of the storyline. Often, she will tell the player what
objective needs to be done next, such as going to a specific building
or destroying an obstacle. Of course, she always pushes the story
along, informing the player of the backstory of events, some of the
lore of the game (such as explaining how Columbia stays afloat
through what could only be described as quantum levitation, or
explaining how the Lutece twins, who are both story driving
characters themselves, exist). She has a direct influence on driving
the player forwards as she can bring in objects from other universes,
such as a skyhook or cover or weapons, that allow the player to
progress further on. To add to that, she is also able to pick locks
at the player's command, which unlocks additional places in the world
as well as the next stage to the game. And in terms of finding
secrets, she can also decode cyphers that occasionally appear on
walls, which lead to additional secrets. Elizabeth's overall goal
changes across the game. First she wants to get away from the main
antagonist Comstock, then she wants to get away from the protagonist
Booker DeWitt after she realises she's been betrayed, then eventually
she turns to wanting to kill Comstock. Across the game, you see her
evolve into a more hardened character. She can take care of herself,
as the game tells you, as she tends to hide in safety when enemies
are about and can pass you ammo, health and money when you're running
low and the aforementioned summoning from other worlds. These
mechanics make her not only an interesting character, but also make
her a useful asset to the player. She is the most important character
for driving the story due to her knowledge, her personality and her
position.
As I said before, you get characters like Elizabeth who not only
drive the story onwards but also help the character directly, and you
can also get characters that direct through narration. One such
character is Wheatley from Portal 2. A comedic character who is met
right at the start of the game, he guides the player from one
objective to the next, telling the player where they need to head to.
As he cannot directly influence many things, he needs the player's
help so he can help the player. This will be things such as
activating switches, or plugging him into a terminal. However his
character is very dim-witted and pretends to know what he's doing for
the sake of dignity (despite being a complex A.I. System). Often,
this leaves the player to solve the mystery themselves, which can be
objectives like cutting pipes with a laser. It's quite a unique way
in driving the story forward as the player comically watches Wheatley
attempt to do things before giving up and wandering off to do it
themselves. He'll do things such as hitting random buttons and
pretending to know what all of them do, or he'll be optimistic every
time he hits something such as getting excited when the lights come
on. He'll also have moments where things will be “a little
embarrassing” and he'll ask the player to turn away for a moment
while he tries to open doors. The first instance, he opens a side
panel rather than the door and the second time he just smashes the
window. These actions not only give a clear purpose and direction for
the player, but also gives the game comedic value. Later on, when he
takes over as the tester of Aperture, he'll start trying to build
puzzles out of scraps in an attempt to keep the player moving, his
purpose being that he “has an itch for testing” now. He's a
juxtaposition of the main antagonist GlaDOS who is intelligent,
sarcastic and uniform in her decisions and Wheatley's complete
opposition adds a certain light to the game that makes it that much
more compelling as the player is driven onwards in two different ways
by two different characters, kind of giving the player both sides of
the coin.
Ref:
“Bioshock Infinite” Irrational Games 2013
“Portal 2” Valve 2011