Harold and Bob, First Numens

Any effects (such as proliferate) that interact with counters a player gets, has, or loses can interact with rad counters.

Rad counters are a kind of counter that a player may have. They're not associated with any specific permanents.

A token that's a copy of Harold and Bob won't return to the battlefield from its owner's graveyard.

In a game using the shared team turns option, such as an Archenemy or Two-Headed Giant game, the inherent triggered ability associated with rad counters triggers once for each player on the active team that has rad counters. Each instance of that ability is controlled by one of those players.

The cards are milled all at once, which means abilities that trigger "whenever one or more nonland cards are milled" will trigger exactly once as long as at least one nonland card was milled.

Rad counters don't go away as steps, phases, or turns end. They only go away when an effect instructs a player to remove rad counters from themselves.