So Guise plays Uh, Yeah I'm that guy, High-5's Fanatic.
Fanatic plays End of Days.
Start of Environment turn Uh Yeah, I'm that Guy triggers first, destroying all of infinitor's Manifestations, A bunch of Omnitron IV's stuff and then destroys End of Days.
At that point the text leaves Uh Yeah, I'm that guy and the effect stops.
Sounds reasonable to me.
Awe. Some. 
We had an amazing game with Guise and PW Fanatic. Also Guise pinging Cryo Bot (at +1 because Champion Bot) with his base power since all villains had -1 taken.
But in the middle of his throwdown he copies Fanatic's base power redirecting the 5 radiant because even a Barbaric Guise couldn't hit himself through that Chainmail Bikini, then put Best Card ever into play from atop his deck (thanks Tempest) and played I think three more cards before his fun ended (for that turn at least).
PW Fanatic is awesome, and gives Guise a great base power to copy.
So, "Uh, Yeah" really does update real-time as cards enter/leave the target's play area? Good to know!
Yeah, it cares what cards are in that players play area. It would update just like "X" updates.
J'Accord!
Not sure if this is something y'all discussed during playtesting, but the portion of UYITG that copies the other hero's ongoings - does anyone know for sure if that text on UYITG considered to be before or after the closing line on the base card to destroy it at the start of Guise's next turn?
It goes in the order on the card, so all copied start of turn text will happen before the destruction on Guise's turn. Of course any copied effect that destroys at the start of the turn will destroy Uh, Yeah.
Which is awesome when you copy The Naturalist, and use it to regain 4HP via Natural-Born Vigor at no additional cost.