Quoted By:
Not telling Keiran was the right decision.
You have to remember a few things, you've just met Keiran a day or two ago. While you quickly become friends, you've barely spent time with him even then, only ever doing so at the monuments because he chooses to go by himself to these locations and only shows up as you do or right after. Meanwhile you meet Ogerpon in the dark wearing a mask which you can't quite make out, for all your player character and Carmine knows, it's a child that has gotten lost or ran off, only the player character has some minor inkling it's a Pokemon. It's only after showing the Teal Mask to the grandpa that we have an understanding of the situation. Even then, the granpa and Carmine both tell you not to tell Keiran. Remember, you're a kid, 12 years old at the absolute most being told by Keiran's literal family not to tell him, a boy you have barely known for a day or two, and for good reason. Look at how he almost immediately acts upon not being told the truth to protect him.
Now to defend Keiran, obviously he's about 12-13 years old too and doesn't quite understand that they're doing it to protect him, just as he doesn't quite get that Carmine is mean as a way of toughening him up and not showing her real feelings and protecting him, which is why he sides with Ogerpon so much as the outcast. But his immediate reaction is still to just constantly battle his so called friend for not being honest with him a singular time, the friend he's again, known for probably a total of an hour or two between two or three days at this point. And despite the parallels between You, Carmine, and Grandpa being the Loyal Three "villains" and Keiran being Ogerpon, he still tries to go against Ogerpon's wishes by proving he himself is better, it's selfish/entitled.
Also imagine what he would have done being told the truth, he'd have ventured into the mountains to catch Ogerpon, potentially getting hurt by being reckless and stubborn by trying to catch it.
