The cast surrounding Marvel’s God of Thunder continues to develop.

According to The Hollywood Reporter, Jaimie Alexander and Canadian actor Colm Feore are the latest additions to Kenneth Branagh’s upcoming Thor. Alexander will play Sif, an Asgardian warrior who can hold her own against any man, and the romantic rival of Natalie Portman‘s character.

Feore’s role, meanwhile, is being kept top-secret, although it is rumored he will be playing a villain alongside Tom Hiddleston’s Loki.

Thor revolves around a mighty yet arrogant warrior who, as punishment for re-igniting an ancient war, is cast down to Earth to live among humans. Along with learning humility and how to be a true hero, he falls in love with a mortal woman named Jane Foster (Portman) and saves the world. Thor is one of several Marvel Comics movie adaptations expected to dovetail into the 2012 release, The Avengers.

Alexander, who played Jessi in Kyle XY, will be appearing in the upcoming Love and Other Drugs, starring Anne Hathaway and Jake Gyllenhaal. Feore, meanwhile, recently appeared in the television show 24, but has also starred in the films Thirty Two Short Films About Glenn Gould, Titus, and The Chronicles of Riddick. It seems more than a little suspicious that Feore’s role is being kept under such tight wraps; will he be the sidekick to Hiddleston’s Loki, or perhaps something more important?

Given how the Canadian actor brings a natural sense of culture and intellect to his roles, we envision him as more of a puppet-master baddie than a brawler, but we’ll assume Branagh knows what he’s doing.

Thor is targetting a May 20, 2011 release date.