Whenever a character enters play this turn, put a +1 /+1 counter on it.

im a bit confused with this card.
does this have to be played from the resource row? or can it be played from hand since its not "ongoing" effect?
also if i were to play it durring my recruit phase off-init would my opponent's chars that came into play this turn, before strategic thinking was played, get counters?

Its sets up a delayed trigger, key word here whenever. It last for the turn as it states and as you state is not ongoing. Can be played from the hand because it does not state otherwise. Only effects characters that come into play after it is played, because only they trigger its delayed trigger effect.

Hope that helps, let me know if anything is still unclear.

Could be interesting with Cable or someone who slides back in.

Well you want to play it off initiative so that your opponents characters don't get he +1+1 counter.
Since the trigger says whenever his characters won't get the counter. However if he manages to remove them from the game to later get them back in play say with something like Cable and Bodyslide then they would get the counters.

Also could be used with the 5 drop MUN that can't be stunned in combat by a character with a +1/+1 counter. I think that trick was teched into a couple of decks during Worlds if I remember the reports well enough.

sweet. it does work as i thought.
and that 5 drop is ultron
ty