Short Thought: Why Games with Deuteragonists Are Great
Deuteragonists are characters that work as the second protagonist. Significantly more important than a side character, but not quite as important as the main character (also known as the protagonist).
Lately, there has been an influx of games including deuteragonists. Each of the ones that I can think of are great, easily topping the charts with their releases. So that we're on the same page, some games with deuteragonists are The Walking Dead: Season One, God of War (2018), and The Last of Us, with each of the deuteragonists present within the games being the child.
It's rather easy to pinpoint why these characters are so great. You see, whenever you're playing through a title featuring only a protagonist, the character tends to be very quiet and a lot of their interactions limited by the inability to portray character thoughts easily. One of the examples of games avoiding this is Aloy from Horizon Zero Dawn, as she'll constantly talk to herself throughout the story, allowing us to create more of a personality out of her aside from her interactions with other NPCs.
Part of what made The Walking Dead: Season One such a great experience was how Lee had to take care of helpless Clementine — the deuteragonist — as you see him struggle with how to approach every scenario with her, as he would have (more likely than not) worked as an anti-hero or typical protagonist depending on which survivors he sides with. Instead, with Clementine in the picture, his character arc unfolds more on protecting the little girl, and forms bonds with father-figures present throughout the game.
Deuteragonists are used to affect the protagonist in many ways and change them, much like an adjective. While throughout The Last of Us Joel acts more distant from Ellie, she works to affect him into a more soft-demeanour personality, much like Atreus does to Kratos.
Deuteragonists ensure that the best (or worst) of the protagonist is accentuated. Without a deuteragonist present, a protagonist is often silent and will interact only with the side characters present throughout the game. This is the reason why deuteragonists — and sometimes tritagonists, such as Mimir in God of War (2018), or Kenny from The Walking Dead: Season One — more commonly appear in story-based games; it helps the player connect to the protagonist if you can hear them interact and talk to other characters more often.
Atreus as a character works to change Kratos from the rage-fuelled Greek god he was throughout the original saga into a caring father that tries to learn how to control that anger. Since now he must care for Atreus, his decisions cannot only be made on whether it affects himself but the deuteragonist as well, discouraging foolhardy ideologies due to the other life they can affect.
All-in-all, games with deuteragonists ensure that we can learn more about the protagonist, despite the shared screen time.