Quote from Craterallus » "For the erratic portal specifically as the controller of the creature you choose whether to pay the 1 or not."
Which, if I am understanding you correctly, is me. I control the creature because I took it over with willbreaker when I targeted it with the erratic portal.
That's an interesting point about targeting with spells which only target creatures opponent controls. So like, a Harbinger of the tides would target a tapped creature an opponent controls but with willbreaker in play I would gain control of the creature targeted by Harbinger of the tides