Well... There's a lot to explain... So I'll just make it as short as possible...
So basically, we had 2 Trollge wars, in which we were fighting a shit ton of creepy Trollfaces, alongside the OG Trollface. After the second great Trollge war, a few days later an actually nice Trollge arrived, apparently he can travel through universes, but there is one universe he cannot enter and that's SMG4's universe, all thanks to Negative, Negative is Zero's partner in crime, he is not letting The Guardian go to that universe to make sure Zero doesn't lose. So basically The Guardian is like a guardian of every universe in existence.