Castiel is an angel who crossed into the world of Scooby Doo because of a ghost boy.
Personality
As an angelic warrior who used to command armies, Castiel is at first annoyed with having to deal with the antics of Shaggy and Scooby who he is moist often paired with. However, Castiel grows fond of the two and thanks them for showing him the great strength of laughter in the face of danger. Castiel is brave as seen with his daring rescue of Shaggy and compassionate as seen with his distress at what the ghost boy has been through and his healing of Shaggy's broken arm to help keep the illusion that there is no such thing as ghosts.
Skills and abilities
As a part-time monster hunter, Castiel possessed greater knowledge of ghosts and how to fight them than Mystery Inc. and was thus able to handle himself better than Scooby and Shaggy when facing the ghost kid.
As an angel, Castiel possessed many superhuman abilities. He was strong enough to effortlessly hold and carry Scooby and Shaggy at once. Jumping from a second floor balcony, a fall that led to Shaggy suffering a broken arm even with Castiel cushioning his fall, left Castiel completely unharmed.
As an angel. Castiel possessed healing powers that enabled Castiel to heal Shaggy's broken arm in a matter of seconds to the point that it appeared to have never been broken.
History
Supernatural
Scoobynatural
Upon his return to the Men of Letters bunker in his own world, Castiel was sucked into Scooby Doo where he followed the Winchesters and the Mystery Inc. and was briefly mistaken as the culprit. Castiel joined the hunt for the ghost and was paired with Scooby and Shaggy to his annoyance due to the pair's antics. After Shaggy fell off a balcony, Castiel jumped after him and saved Shaggy's life, but Shaggy broke his arm, something that had never happened before despite all the danger that Shaggy had faced. Castiel worked with Shaggy and Scooby to trap the ghost and was saddened to learn of the truth behind its actions. Before returning to his world, Castiel used his angelic powers to heal Shaggy's injuries and thanked Scooby and Shaggy for all that they had taught him.
After returning to his world, Castiel joined the Winchesters in putting the ghost to rest and bringing the bad guy to justice using the man's tax evasion to have him arrested.