When Castiel absorbed billions of soul inside him in order to defeat Raphael, he also absorbed the monsters known as Leviathan who were locked away in Purgatory eons ago. The Leviathans tried taking over his body so he sought the help of Winchesters who re-opened purgatory to send back all the souls, but the Leviathans stayed back, causing the deterioration of Castiel’s vessel and causing his demise in Season 7. They eventually broke free from the body of Castiel.
In Season 11, when Darkness broke free of the captivity, Winchesters were trying their best to defeat her. They needed all the help they could. Castiel again having the best intentions ended up saying YES to Lucifer who would then possess him.
While the Shadow AKA Empty has no form, this cosmic entity could take up the form of anything or anyone. When Castiel woke up in Empty, this entity took up his form to communicate with him. This is a dimension which existed long before the darkness and God. The souls of all the supernatural beings such as angels and demons arrive here where it sleeps forever. So when Castiel had awakened and gained consciousness, the Empty had to come out of its slumber too which annoyed it deeply.
Jimmy Novak has made his appearance in a few episodes. He is the true vessel of the angel Castiel. Jimmy is a man of faith who was destined to house the angel Castiel. When Castiel communicated with him, asked him to prove his devotion, Jimmy complied and impressed Castiel. When Jimmy Novak was mortally wounded, he was concerned about Castiel possessing Claire who is just a child. He asks Castiel to possess him again, knowing the consequences that he will forever be tied to the angel. However, in Season 4 finale, Castiel is blasted by Raphael, both Castiel and Jimmy died. But Chuck brought back Castiel and repaired the vessel of Jimmy Novak. Since then, every time Castiel died, he has been resurrected in the Jimmy’s vessel.
The vessel of Jimmy Novak is very strong that it managed to house an archangel Lucifer and an angel both at once without exterior deterioration. The only vessel in SPN Universe which also housed a demon, an angel, and an archangel all at once. The form of Jimmy Novak is the favorite of Castiel as we have seen him manifesting in this form even in the realms of Empty and Heaven where he could have chosen any form.