Erik Selvig is an astrophysicist and mentor of Jane Foster and Darcy Lewis. Dr. Selvig is a Professor of Theoretical Astrophysics at Culver University. His notes on thermonuclear astrophysics are key to understanding the Tesseract.
Selvig was a very brilliant scientist, helping Jane discover Thor when he first landed on Earth. Selvig was devastated when S.H.I.E.L.D stole his research. He later helped Jane break Thor out of captivity, lying through his teeth to Agent Phil Coulson. He helped Darcy, Jane and The Warriors Three fight the Destroyer when Loki Laufeyson sent it to earth. He was seen at the end of the movie, assisting Foster and Darcy in their attempts to open a portal back to Asgard.
Selvig also featured in the post-credits scene of Thor, when he was taken by S.H.I.E.L.D to meet Nick Fury. Fury showed Selvig the Tesseract, saying that it could contain unlimited power.