Let's suppose we have elements of...
-Sharing a tent by themselves, cuddling to sleep, sharing earbuds, whispering secrets to each other, and generally maintaining about as constant physical contact as a pair of cats.
-Reluctant ride or die energy.
-Being, out of the playable cast, uniquely unworried and already accustomed to the rules of the magical post-apocalyptic setting that they just found themselves in.
-Unhesitantly eating lethal blows directed at the other.
-Calmly getting to work and barking clear and precise orders at party members when the other is unconscious with grave wounds to be mended.
-Mildly seductive remarks specifically directed at the easily-flustered other.
-Silently shushing party members before they ignorantly ask a question that serves as the setup for the other's joke they've heard several dozen times before.
-Physically tearing apart and rebuilding the other's body to fix years of debilitating permanent injuries.
-Regularly causing the token sane adult to become deeply concerned.