Eric Masterson originally appeared as a supporting character. Later, he was gravely wounded was merged with Thor, a la Don Blake. Later the two were separated, but Masterson was taken prisoner by Ulik and Loki. Thor seemed to kill Loki during the rescue. As punishment, Thor was banished but Masterson was permitted to serve as Thor. Most were unaware that he was not the true Thor. Masterson often made comments that he did not like deceiving others into thinking he was the real Thor.
It wasn't Eric, but Kevin (his son) who was kidnapped by Loki. Thor and Eric weren't separated until after Thor was banished and then Eric went out of his way to find Thor and bring him back.