Our game is, at least at present, pretty darn opaque about backstory. We haven't provided information about the characters, their relationship, or their purpose, and that's deliberate, so information we impart about the characters needs to be done through gameplay.
We give hints about our characters through their abilities, which are differentiated according to their personalities. For example, our boy is given the use of a lantern whose light changes the environment:
This ability is central to how the boy plays, and to his character as well: he is first of all perceptive, able to see things that others can't. Underneath this are related personality traits: creativity, imagination, thoughtfulness. When exploring a new area it makes sense to traverse it thoroughly with the boy to uncover hidden secrets.
At the same time, his lantern sometimes reveals obstacles:
He's creative, but also a little stubborn or even pessimistic, constrained by imagined (if not imaginary) problems that only he can see.
Meanwhile, our girl is able to jump a little higher than the boy:
These abilities also inform the characters' relationship with one another, while at the same time illuminating one of the main themes of the game: separation.
Between the lantern creating paths and obstacles, and the girl's extra mobility, many of the game's puzzles require that the characters separate from one another to proceed. In the puzzle below, the girl and boy are separated when the girl opens a door for the boy that she cannot follow him through:
|Insert light-related pun here|
The above are provided as examples of a concept that I feel is important to game design in general: using the gameplay to inform the player about the story and about the characters. When I spoke to playtesters about the game, I found that many of them became quite uncomfortable at the moments of forced separation; they felt like the characters belonged together, and tried to reunite them as quickly as they could. This feedback has informed other design choices: tweaking levels so that no character may progress too far without the other, changing the display so the characters can see more when they are united and changing the sound so that music fades or becomes more sinister when they separate.
The more text or cutscenes used in a game, the more emotion or mood can be conveyed using voice acting, or word choice, or animation. However, even in a game using these tools, gameplay is an important way to help the players feel involved and invested in these stories, and ensuring that your mechanics and controls inform the player about the world they're inhabiting is an excellent way to make that world more attractive to spend time in, and more memorable.