Can you use Leaf Brooch's effect on the turn you play it if you have already played an event that matches sphere? For example, I play Daeron's Runes and draw a Leaf Brooch, attach it to Pippin, and play another lore event. Does that second event gain secrecy? Leaf Brooch applies to the first event each round, but does it have a memory that spans before it enters play?

There was a similar question about oppressive forest that said you can only play one card per round, and Caleb ruled that the limit applies retroactively. So, I suspect the same is true with Leaf Brooch (i.e., it doesn't work on the second event), but I figure I'd ask because the language is different.

I would say that it is, indeed, only the first event. The memory is retroactive.

.....no idea.

Because you point out the rule on oppressive forest, I'm with DukeWellington here, there is only one «first» event you play of the matching sphere.

The idea is to limit Leaf Brooch one per round (as if it must be exhausted to gain the secrecy 1), so i would say that in that case there is not retroactive effect.

It would be very easy to just say "exhaust it to give the next event secrecy". They didn't, therefore, what they DID say has to be taken as is, not for what you assume it is intended to mean.

It's the first and only the first.

There shall be no second or third for this rule.

Because they are not first, otherwise they wouldn't be called second or third.