"...I should probably just have Kimahri give you a crash course in combat spheres."

"...hm."

"...later?"

"No. It is important."

The gist of it is that combat spheres are condensed power leftover from defeated fiends. Spheres in general are a natural way for pyreflies to arrange themselves in, whenever someone does the work to condense them at all, and the term "combat sphere" refers to those you extract from fiends when you use a certain special spell to do so.

By default, if you literally just grabbed a bunch of pyreflies from a dying fiend and condensed them into a sphere, you wouldn't have anything useful at the end, and you'd more likely than not end up accidentally grabbing the pieces of the souls of the dead instead, which no one wants. What this spell does is separate and order the dissipating pyreflies into useful pieces, defined sort of conceptually in intuitively straightforward ways. And more powerful fiends have more pyreflies tacked on in addition to the souls that make them up and so they typically can generate more combat spheres at the end. And then those combat spheres can get absorbed into your soul and expand your capabilities; that is the main way in which one becomes stronger.

However, you can't just graft spheres onto your soul without limit. It takes a while for the power to become truly yours, rather than borrowed or added on, and if you try to absorb too much all at once you'll end up just losing most of it; your soul won't be able to hold onto it without losing hold of itself and holding onto itself is its main job. The maximum amount of power you can safely absorb at a time is known as your "temporary maximum power", or informally "temp max". The best and quickest way to fully integrate spheres you've absorbed into yourself is through use. Typically, that just means combat, but it doesn't need to; you could in theory absorb a Power Sphere just as well by playing blitzball. It's just that eventually blitzball stops being a challenge in the right way, so you're not really meaningfully using any further improvements you get and so integrating them is slow. Fiends, on the other hand, can become unboundedly powerful, which makes combat always a good way to do it.

That spell is not, technically speaking, the only way to acquire combat spheres. You could make ones the same way you make any other spheres: gather up enough pyreflies, apply your will to them, the end. What that spell does, though, is make this process be very streamlined. Other ways of creating spheres require effort and care and thought; this spell makes use of the extant coherence of fiends, who are already solidified pyreflies themselves and who are already using those pyreflies for their own power, and just redirects that power. It's very efficient and effective, and creates a useful loop for field combatants.

As for the colour of the spheres, those are just built into the spell for ease of categorisation: red ones are standard spheres that improve you in one way or another, yellow ones are sympathetic ones that allow you to share power with someone else, and purple ones are extra-reinforced in a way that allows you to absorb them even if you're already at your temp max because they do the work of staying attached to your soul until fully integrated on their own. Which kinds of spheres a fiend drops is kind of complicatedly determined by the fiend's power and, in a manner of speaking, "personality", since all of magic is so emotions-based. Purple spheres are very rare, whereas red spheres are the most common.

That explanation gets broken up by fights with fiends but even though they're numerous and kind of stirred up by Sin they are not, individually, very strong, especially compared to their party. The main handicap they have is protecting the blitzballers, who don't have any kind of combat experience whatsoever. It's still not particularly difficult.

The temple they're going to is at the top of a hill that is not tall enough to be called a mountain. There are stairs leading up to it, some fifteen to twenty flights of them separated by circular landings, most of them reasonably small but one large enough to serve as a good rest stop with some benches and a great view of the village and the ocean, halfway up. But despite not being a mountain it is still very, very tall, and will require Rather A Lot Of Climbing.

"Damn," sighs Zei when they get there, having to shield his eyes from the sun with a hand as he looks up. "I don't suppose we could take our lunch break right here?"

"It'll be a lot harder to climb with a full stomach."

"Hey, hey," says Letty. "Fancy a race up?"

"You realise Lord Mi'ihen used to go up and down these steps to train stamina, right? No, I do not fancy a race."

"Lord Mi'ihen? The guy who founded the Crusaders?"

"The very same."

"You know what, why not, I'll race you guys. To the middle landing?"

"Yeah! That's the spirit!"

She sighs exasperatedly.

All of the blitzballers seem keen to race, so they all line up side-by-side at the bottom of the stairs and start stretching.

Zei rolls his eyes with a small smile playing around his lips but then walks over next to them to extend an arm horizontally in front of them as a stand-in for a flag. "On your marks, then."

They all assume starting positions, then.

"Ready... set... go!"

They go.

"So since the three of us are the only sensible people here, let's go up at a reasonable, sedate pace, why don't we?"

"Hm."

When they reach the landing the whole team is various shades of dead, with Keepa and Botta literally lying facedown on the stone floor, panting heavily.

Wakka and Azym look to be in best shape amongst them, and from Wakka's smug look he probably won.

"Congratulations. Now shall we climb the rest of the stairs?"

"Please... just... five... minutes..." says Keepa, gasping.

"You guys chose to race, deal with the consequences," says Azym, who is mostly just winded.