Need help with a backstory for my street brawler/wrestler monk

I think your issue comes from not having an accurate concept of Nihilism.

“The universe doesn't care what we do, but what we do still matters.”

That isn’t Nihilism. Nihilism has nothing to do with reality, just one’s outlook on reality. Nihilism is:

“Nothing we do effing matters so why bother? Do whatever the eff you want because nothing matters. Eat healthy, eat unhealthy, eat babies, whatever floats your boat. None of us are ‘special,’ we’re all just walking talking piles of slowly decomposing meat anyway. We might as well kill ourselves but even that wouldn’t matter so we might as well not.”

That is Nihilism. The reality may be that everything matters and that nothing is inconsequential. But the perception of the individual is what nihilism is about.

I'm basing people's Nihilism coming from a viewpoint of evidence and not assumption.

This relates to the OP basis because the character may have come to realize that a butterfly beating its wings starting a pattern that creates a hurricane makes everyone else's plans and hopes completely moot for forces that give no consideration for anyone.

Why did the character shirk the family tradition? Tradition or its supposed importance to the family meant nothing in a grand scope. Nihilism.

The growth is understanding that having no affect on the universe as a whole - the total perspective - isn't what's important in the end. ...but how does one take that character, who decided to give family tradition the finger, to go do something greater than the person thinks is possible?

Where do we put that seed of doubt in Nihilism - a belief that makes even doubt pointless? Without that kernel, the character can't grow.

You miss the point. Nihilism rejects the butterfly effect as even that butterfly’s actions are completely irrelevant to the universe. No butterfly can create a hurricane, it’s all just random crap. Belief that a butterfly could affect the world in that way would run in direct contradiction to the concept of Nihilism. To a Nihilist, all “evidence” points to the realization that “absolute nothing matters whatsoever, it’s all just random meaningless crap.”

You know that “optimistic nihilist” I described earlier...? Hi, my name’s Sposta.

I want things to matter, I want things to get better, but four decades of evidence has proven that nothing matters, and never will. In another hundred years no one will even remember our names, let alone our actions. And what I do today will only inevitably be washed away by the passage of time until utterly forgotten and the world has to do it all over again. So I keep doing things based on my innate belief that my actions will potentially make a positive difference in the world... until the world yet again proves that hope to be utter garbage for the bazillianth time this year alone. “Sonofa*****, I should have known better! Why do I keep doing this to myself?!? I’m such an *******.” And of course the cycle repeats itself because of an innate optimism. The optimist starts from a position of positive outlook that the nihilism crushes.

The reason you’re having difficulty with the concept, is because you’re actually trying to describe something different. What you want is the exact opposite, someone who starts from a position of negative outlook, and then becomes uplifted by events. I believe what you’re really going for is “cynicism” or at least “pessimism” more than nihilism and instead of optimism. The person doubts that anything matters, only to be ultimately proven wrong by the universe and change their mind accordingly. You want to start low and end high.

The only way your concept could possibly apply to an “optimistic nihilist” is if the story starts much later in that character’s life after the nihilism has completely taken over, and then have the spark of optimism rekindled through an “inciting incident” that disrupts the “steady state.”

Quote from Spookytrombones >>

I've got the basic concepts down

-parents wanted him to be a scholar or wizard

-decided that fighting was more fun

-ran away from home to fufill his need for said fighting

 Go watch Jet Li's Fearless.

As a follow-up: Han Solo would be an excellent example of a nihilistic optimist.

When asked, he professes to accept that the galaxy is just a steaming pile of refuse and nothing anyone does can ever fix that. Shoot first and get paid, ‘cause nothing else matters. Very nihilistic.

But when the chips are really down, and his back is against the wall, “Never tell me the odds!” Very optimistic.