I think the notion of the importance of non-combat characters in combat stories breaks down if you consider the character in question needs to be the most decisive figure in the story to play a role. Sort of taking on the story from your standard young adult angle where-in the main character of course has to physically combat the main villain or the central problem which is the narrative focus of the RP or story in question. But I think once you break through this limiting factor you can open yourself up to more possibilities than someone being just able to fight a something.
And the most fundamental question to being with is: what is the thing about the character I want to focus on, and how will it change him or her.
At the core of the idea is to imagine the character as himself having his own story, where while intertwined with the stories of other characters in the same universe or RP could be read as independent of all others. In that sense, besides simply filling out the application and stepping in looking to take events of the RP in stride as they happen is that you step into the RP with a character and a goal in mind for that character. How do you want that character to change by the end? What do you want to achieve? What's his or her conclusion? Is it a tragic death or suicide? Is it to acquire something new and meaningful, a life improvement? Figuring out what you want to achieve in the context of the RP other than "defeat the Big-Bad" or "Acquire the group objective" sets something complimentary to the story that allows you to focus on something else and won't lock you hard-core into the big meta-objective of the entire party of the entire RP.
Answering these questions will give you the final destination you want to arrive at. A motivation. The motivation for action doesn't have to be a spoken thing and can be unspoken, something unrealized by the character itself. Or the character can speak of some other motivation and arrive to a new one. Perhaps your character talks about finding out why a close family member has died and to what, but along the way realizes something else greater? From real literature, in Kenziboru Oe's book Death by Water (originally Sushi in Japanese) the main character intends to write an fictionalized account of his father's death and wants to look into the mystery and life of his father, to do so he has to access his father's red leather chest which his mother kept from him because she disowned him over an earlier novel, but on her death early in the novel he gains access to the chest through his sister who forgives him and he begin his work; but a quarter of the way through he doesn't learn anything and the entire focus of the novel shifts to something else completely.
A shift of focus just for you and your character to a different objective, autonomous of but dependent on the main RP objective may give you another option to write for, but you'll need to consider the interim in the meantime. On the way events are going to happen outside of your character and you'll need to decide how to engage with all of that in the meantime. One in an interesting way and also in a meaningful way. It may not perhaps be best for your absolute pacifist to spend the entire time with the main party and he'll have to peel off tactically at times to do something else while everyone fights cave goblins; there's after all far less ways to write about hiding behind a rock from injury as there are ways to write about fighting monsters. Period. You could probably think of this like Gandalf's coming and goings from The Hobbit or Lord of the Rings where he disappears from the focus of the story to go do things of his own that have impact with the story, investigating some mysterious circumstance or meeting with someone else which will bring things into the story.
Since the points between starting and ending are going to be the biggest area you spend time with your character in, this deserves the most meditation. Even in the step-by-step of Roleplaying. But also comes back around to the intent of the character. Referring back to my previous example of Death by Water: how does the relationship of the character with his Red Leather Chest change in the story, or how does it become abandoned for something else?
Another way to think about things might be how far can you get away with thinking not just about the character but the group as a whole and what sort of circumstances can you bring to bear on the group through the power of the pacifist character. If he's not going to be fighting the biggest dragons, how can his activities help or indirectly hinder their progress or just over all help with their development as a whole. To this I turn to a story I like to bitch about these days. A Fan-fiction actually.
Fallout Equestria is a crossover story between My Little Pony and fucking Fallout. I do like the concept for its juxtaposition of values and aesthetic it forces and apparently a lot of people do too within the MLP fandom. Or enough so that after the completion of and even during the writing of the original Fallout Equestria story a plethora of alternative side-stories bloomed. Some ignored, others praised as being central to the blooming sub-fandom's central canon. One of those is Project Horizons, a story which tries to take everything about FoE and crank it up to eleven. The main character gets into more fights and battles than the original story's MC and commits far more atrocities on her own than the original. And while the story attempts to come to terms with those, its never a complete exploration I feel. The story would have at the least benefited from someone to attempt to help the main character rationalize this shit; even if she proclaims herself to not be at all smart.
Setting aside any detailed analysis for how that could have been done, let's get to the point I'm making with that: in the course of the RP's story how does the non-combat character develop themselves but how does their relationship with his or her peers assist in the development of the other members. Does their existence intentionally or unintentionally create drama between them? Do they hurt or heal them? Can the existence of the pacifist help to conduct the changing values of the others to an "evolved" state in the end, not just physically stronger but also emotionally and psychologically more matured? And how does the rest of the group add to the development of the pacifist? Do they in turn encourage directly or indirectly, spoken or unspoken the pacifist/non-combatant member of the group to take up the gun or the sword and reshape their own relationship to violence and physical action?
A pacifist, non-fighting character is not without their own dynamic and fascinating challenges. They're not fighting external dragons so to say, but grappling with internal ones. And I think the idea of to write them and their own importance needs to shift from external exertion to something more internal. How also might the story be read if just from the pacifist? I've suggested their own goals, which may be their own plot and with their own conflict. In an RP environment you have the opportunity to unravel everything into their independent strings and read it as a story of each of the individual characters as you can re-weave it to be the story of all the characters going to achieve something as a uniformed braid of rope again.
