Amica Endura

You have to open your heart to others because Transformers hate metaphors.

On both Cybertron and Caminus, an Amica Endura is a Transformer's long-term best friend. For a species that can live for millions of years, that sort of lasting friendship is a big deal. On both worlds it involves a formal ritual, baring your spark and inviting someone to "receive my light" before explaining why you value them.

A related-but-distinct concept is the Conjunx Endura, the equivalent of a spouse.

Fiction

2005 IDW continuity

While a Camien Amica can become a Conjunx, on Cybertron you have to rule out being someone's Conjunx before "opting" for Amica. Cybertronians are sick, man. Your Fierce Tears

Nightbeat questioned Nautica if she had a Conjunx Endura, any Amica Endurae, or if her homeworld of Caminus practiced any sort of elective kinship. The answers were "no", "yes", and "yes". slaughterhouse It turned out on Caminus, you had to get an Amica by ten megacycles or you were generally shunned by society (though not killed, as Skids feared for a moment). Nautica and Firestar formed an Amica couple out of mutual desperation, though their relationship was strained. The Sensuous Frame Thankfully, it got a bit better. The Frail Gaze

On Caminus, the Oath of Constancy is said to form an Amica Endura. The Frail Gaze

It is possible to split up with an Amica Endura on Caminus, known as "Platonic Severance", but, as Nautica explains, rejecting an Amica, even one taken out of desperation, represented failure, failing to do enough to (presumably, as Nautica is cut off before finishing) make the relationship work. Firestar later suggests that her and Nautica end their friendship, but Nautica refuses. The Frail Gaze

Aileron, another Camien, was about to describe Sterling as her Amica Endura to Optimus Prime when she stopped herself and went with the Cybertronian equivalent, "best friend", instead. Aphelion

When facing certain death, Nautica decided to open her heart and admit how she felt to her closest friends. Gathering Velocity, Skids, Rung, Brainstorm, and Nightbeat together, she opened her spark casing and recited the words, as everyone joined hands and accepted. At Close of Day

Ask Vector Prime

Stealth Saber and Victory Bomber were Amica Endurae. Spacewarp's Log (1), 2015/10/27

Notes

• Nautica remarked most people would sidle up to those questions, making it rather gauche to ask outright.
• "slaughterhouse" did not explicitly say what an Amica Endura was, leaving it up for us to work out, but it was confirmed later by James Roberts. Later, in "The Sensuous Frame", Firestar defined the term as "best friend".
• The concept bears some similarity to the human tradition of ceremonial blood siblings.