darkSpyro

Just confused on how this works.

For example, if I am playing Trap Team and buy Trigger Happy's Soul Gem power, what happens when I put him in Spyro's adventure? Is the soul gem counted as collected, even if I didn't collect it yet?

He'll have the upgrade on him and you're able to use it, but it wont unlock it in the game.

Thank you.

I wonder what happens if I buy it in Trap Team, go back to an older game and then switch paths. Think it will still let me upgrade without finding the soul gem?

If you bring figures from SA, Giants or SF to TT, you don't need the Soul Gem. You just need the gold to purchase its power like any other upgrade.
The only time you need to find the Soul Gem is if it's a new character in a new game.
For example, if you find Blades's Soul Gem in Soda Springs, all you need is the gold to purchase it as well as any required previous upgrades (if applicable). What that means is that some Soul Gem powers may need an earlier upgrade to unlock the Soul Gem's power.

What others have said - the Soul Gem is a collectable item relevant to the completion percentage on a particular save file. You can take Trigger Happy to a friends house (or even just another save file) where the soul gem is already collected, do a full upgrade, then return to your regular save file with the ability still unlocked on your figure. But you'd still have to collect the soul gem for that save file.

A side question - in Trap Team if you buy one upgrade path on a series 2 or higher figure that can switch paths, the game gives you the other path for free. Whereas in Swap Force and Giants, if you switch paths you have to pay for the other 3 abilities as well. So if I upgrade series 2 Trigger Happy in Trap Team and take him to a previous game, is he considered 'paid for' on both paths if I attempt path-switching there?