Blood Siblings

For notes on the relationship akin to human siblings, see Litter Mates Since the earliest pre-Unification conflicts, Stenza have recognized the bond that the fires of war forge between people. Peculiarities in Stenza psychology mean that this bond most commonly forms between pairs, although trios are not unheard of. One traditionally refers to one's counterpart in this relationship as their "swlê". Pre-Unification, such bonds formed mostly between members of the same clan and occasionally with members of an allied clan in any given conflict. However, the greatest revolution to the concept came during the Battle with the Stone Menace, an existential conflict between an unknown creature and the entire Stenza species. By the end of the war, all Stenza clans had been fighting and dying alongside one another, protecting each other's children and securing each other's food. This vast web of bound relationships was unprecedented, and ultimately one of the greatest contributing factors to the initiation of lasting inter-clan peace. The swlê serves a wide variety of social functions for their compatriot, including but not limited to:

being a mediator in their love life (this includes both playing wingman for the partner and telling a poor, love-struck sop when it's time to cut bait)
serving as secret keeper (it is so entrenched that bound pairs will not testify against each other for any reason that most Stenza authorities have long since abandoned even hoping to bother with it)
serving as protector (both physically and socially; in modern times, Ta'zhen has received substantial cover due to her bond with Tzim-Sha, a widely regarded (if still controversial) First Officer)

The bond is held to be nigh unbreakable, although almost no one known has ever tried. It's widely feared that if one party in such a relationship breaks rank, as it were, it could cause a ripple effect that cascades through all of Stenza society, and the Unified Clans would collapse back into pre-Unification civil war. (Cooler heads, however, think inertia may win the day at least in the first few years or decades, but no one is prepared to take that big of a risk.)