What is the affinity that makes this Pacifist faction unique to play? 

Defenders of Life - After the great mistake the Gorumà saw the destruction and loss of life they caused. After this they vowed to protect life in all its forms. Gorumà get a militaristic bonus when joining a war against an aggressor to protect a smaller faction. They also gain a bonus to diplomacy with that smaller faction they protect / A boost of influence. The Gorumà cannot declare war for any other reason and cannot refuse a peace offer that includes peace with the faction they are protecting. The Gorumà also get a bonus to diplomacy with all factions / an influence boost per number of planet types they control and number of plant / animal planetary bonuses they control. (Think the Tree of Life.)

What is the story behind this faction?

When the Gorumà  first made it into space the entire species was ecstatic. What was once only a dream became a real tangible possibility. That dream was short lived and soon because a nightmare. Before even colonizing their first planet the Gorumà  ran into another space faring race. Unable to make proper contact the military leaders of the Gorumà  fell back into their primal past and opted for open war. What was expected to last years ended in days as the Gorumà  completely obliterated the race they had met. The Gorumà  razed their planet so thoroughly it because a tomb world devoid of all life. After inspecting the carnage the Gorumà  saw what they had done. The planet they had destroyed in their xenophobia was once a vibrant forest paradise. The race they had killed was peaceful with nothing more than the basics in defense. The Gorumà were united in the grief of what they had done and the lost possibilities. Soon after they vowed to protect all life they found in the galaxy if they could and to do everything possible to avoid loss of life in any form. They knew what they had done and knew others could make the same mistakes. They would not suffer the great loss of life they had caused again even if it meant they would have to take up arms again.

Looks / Personality

The Gorumà are a big Gorrila looking race that obviously have extreme physical powers. They were once a clan based race with deep roots in warfare and fighting. This inherent warlike temperament followed them into the modern age. While tempered anger and resentment always lurked just behind every dealing. Even now as the Gorumà sail the stars it takes great restraint for them to not fall back onto their roots. This causes them to look hard and speak in short concise sentences. Even so they are dedicated to their vow and genuinely want to help other races and promote peace above all else.

Rational

I dont like fully locking options from players with things such as "Cant make peace" or "Cant declare war" This gives me a happy middle. The Gorumà cant expand militaristicly on their own but at the same time wont always have nothing to do with their units. Its very annoying to me to build a defensive army and them never use it just because no one ever declared war on me because I had such a large army. This way I have the option to fight militaristic civs to keep them in check rather then being stuck as the last one they fight. It is frustrating being unable to stop an expanding civ just because they know you can only fight them when they are ready to take you down.  Now if you see a civ consuming lesser factions you can step in to do something about it while still keeping to the pacifist idea since once the militaristic civ has had enough and wants total peace you cant refuse.

I dont know what the final game mechanics will be so the ability is open ended. The idea is to expand into key planets to gain a large boost towards the diplomacy victory while still having short term benefits. From there its a cold war situation. You want to be big enough that others don't want to fight at all least they risk your wrath. The Gorumà enforce peace not because of their inborn nature but because they know what the outcome of war is. They understand the weapons and power the space faring races have and want to create a situation where no one uses that power.

I like this concept! I think giving them a motivation to go to war is a bit against the pacifist theme though. Maybe something closer to being able to garrison powerful defender units in minor faction outposts would be more thematically appropriate?

A cool concept. "Defenders of the weak" is a neat idea not yet seen in the ES lore. I want to know more about the traits so check out the Game Design Documents, for ideas on how to design ones that encourage diplomacy. Maybe they can defend the weak minor factions instead of the major ones? Also, if they can't declare war outright, how can they expand and/or pressure other nearby factions? I try to balance this idea in my submission, but I feel your concept has a lot of potential. You've hooked me, now real me in!