Like most angels, Castiel expresses little to no emotion. Despite this, he does not display the blank apathy or mocking superiority complex that seem to characterize angels such as Uriel or Lucifer. His lack of emotion and understanding of human traits such as sarcasm has often led to humorous situations throughout the show and have occasionally almost got him into trouble with humans, ultimately leaving Dean or Sam to amend the situation.

Unlike many of his brethren, Castiel still has faith in his father, though not to the near zealot-like level displayed by Michael. While he admits he may not always understand what God wants and question the reason for it, he would always follow his father to the best of his abilities. This, along with his interest in humans, has caused other angels to label him as odd or naive. Anna and Lucifer have both demonstrated this, with Anna calling him a hypocrite for refusing to rebel in order to gain more freedom. Castiel frequently exhibits what could be referred to as friendly affection towards Dean, and seems to at the very least come close to expressing regret, hesitance, and anger several times.
Discussions with Uriel suggest the pair were children by angel standards when Lucifer rebelled against Heaven and refused to bow down to humanity. While his brother was instantly enthralled by Lucifer's power and charisma, Castiel remembered being frightened and disturbed by the rebellious archangel. Castiel had been stationed on Earth for at least 2,000 years while being in the same garrison as Anna Milton, acting as his superior, and Uriel. Castiel was there when the world was first created. He watched as the first fish emerged from the ocean. He also watched them build the tower of Babel, among other historic events.
In 2008, Castiel speaks with his true vessel, Jimmy Novak, in Pontiac, Illinois. When Jimmy is watching television, Castiel's voice distorts the picture and causes him to scream in pain and his body to spasm until he finally blacks out. Later, Castiel would tell Jimmy to put his in arm in boiling water and that he would be unharmed as a way to prove his faith. With his wife believing that he's going crazy, Jimmy asks Castiel what he wants from him and how he can help. Castiel explains that he needs to use Jimmy as a vessel, to which Jimmy agrees to as long as his family is protected. After receiving consent, Castiel possesses Jimmy. Before he leaves, Castiel is confronted by Jimmy's daughter, Claire, and tells her that he is not her father.

Powers and abilities:

Since his first appearance, Castiel possessed astounding abilities and was one of the most powerful beings the Winchester brothers have ever met at the time. Uriel seemed so sure of his powers that he felt confident that if Castiel joined him and his fellow rebels, they would be powerful enough to challenge Heaven and free Lucifer from his Cage. And even Anna, despite having once been Castiel's superior, chose not to directly confront him after Castiel appointed himself as the Winchesters' guardian, and instead went back through time in an attempt to ensure that Sam and Dean are never born. Even when not at full strength, he was shown to be a very capable fighter due to him killing several of his angelic siblings when he was fighting to prevent the Apocalypse. Castiel displayed the following powers when he was a mere angel:

Angelic Possession - Castiel needs to use a vessel while on Earth. As an angel, he needed the vessel's permission.

Astral Projection - Castiel was able to project the shadows of his wings for Dean.

Chronokinesis - Castiel can travel forward and backward in time, and he can take passengers.

Eidetic Memory - Castiel has the face of every prophet etched into his memory.

Healing - Castiel could instantly heal the wounds, diseases, and toxins of humans.

Immortality - Castiel as an angel doesn't age.

Invulnerability - As an angel, there are very few things able to kill Castiel.

Killing Touch - Castiel is able to kill lower-level demons and other beings like monsters with a simple touch.

Resurrection - Castiel brought Dean back from the dead.

Sedation - Castiel can cause instant unconsciousness in humans.

Soul Reading - Castiel is able to determine the state of someone's soul. He is also able to use a soul as a power source.

Superhuman Stamina - Castiel never tires, and does not need to eat, sleep, drink, or breathe to sustain himself.

Superhuman Strength - Castiel is physically stronger than humans, lower-level demons, and most monsters.

Supernatural Perception - Castiel can see things hidden to humans.

Telekinesis - Castiel can impart kinetic energy on matter with his mind.

Telepathy - Castiel is able to read the minds of humans and lower-level angels.

Teleportation - Castiel can travel from one place to another instantly, without occupying the space in between.

Voice Mimicry - Castiel once perfectly imitated Bobby Singer's voice.

Invisibility - Castiel is able to become unseen by humans.

Memory Manipulation - Castiel can erase, restore, alter, and fabricate memories.

Pyrokinesis - Castiel can generate, manipulate, and extinguish fire.

Reality Warping - Cas, in one instance, altered reality, by shaking a 'Sorry' board-game, and having everything instantly set up.

Holy White Light [after obtaining millions of souls from Purgatory ] - Castiel can create large amounts of energy.

Highly Advanced Reality Warping - Castiel was able to project his desires into the physical or spiritual world(s) as he killed thousands in Heaven according to Hester. Castiel could kill humans with a thought or word as well as perform miracles like making the blind see. Castiel even told Crowley when he summaried that Hell was being downsided, that he would have done away with it completely, a spiritual realm. However, even when imbued with the power of nearly 30-40 million souls from purgatory, Death mocked him calling him a mutated angel. Death told Castiel he had met God, and that Castiel was certainly no God. He was nowhere as powerful as Death as the latter could have killed him. Notably, Castiel was concerned with keeping Michael in Lucifer's cage, but as Castiel gave up the souls he had absorbed it's unknown how his soul-enriched powers would compare to Michael's.

Invincibility - Castiel was immune to almost any form of harm (though Death and God could have killed him).

Molecular Combustion - Castiel killed Raphael and caused his vessel to explode by simply snapping his fingers.

Family:

God - (Creator; Father)
Michael - Brother; superior
Raphael - Brother; arch-enemy
Zachariah - (Former Superior/Teacher)
Anna, Uriel, Balthazar, Hester, Inias - (Fellow Soldiers in God's Army)
Other Angels - (Brethren)
Sam and Dean - Surrogate brothers
Daphne Allen - Wife (as Emanuel)
Jimmy Novak - Vessel