Precious things

Gordon as the unintentional war hero that constantly goes through hell but that acts very goofily and almost like a child due to his energy and curiosity whenever he gets the chance, and Chell as a rather average person that prefers to stay serious, focused and with a cool head while going through everything that Aperture has to show to her.

I think those are common headcanons since they fit pretty well with the gameplay style each game tries to push you into, and the thing that makes me like these interpretations the most is the contrast they create, not only with eachother as protagonists, but also with the world they are in.

The overall goofiness of all the shenanigans Gordon goes through while alone makes the lowest points during the Black Mesa incident and the inhuman conditions people live with under the Combine's rule all the more painful in comparison, and Chell's no-nonsense attitude with her lack of reactions give Aperture Science complete freedom to be as ridiculous as it wants. It's like instead of having a secondary character to be the foil of the protagonist, they treat the world as a character itself, and make the protagonist be its foil.