Guardian: The Lonely and Great God

• Gong Yoo as Goblin / Kim Shin [9]

A 940-year-old immortal goblin and protector of souls who is in search for his bride, who is the only one who can remove the sword piercing through his chest. Once the sword is removed, he may finally move to the afterlife and rest in peace. He served as a General during the Goryeo era, where he was appointed as the regent with his sister being betrothed to the young king Wang Yeo. However, due to corruption by a eunuch, the king's guards killed Kim Shin's sister, soldiers, and household before ultimately killing him with the very sword that is lodged in his chest. As both a reward for the good he had done for his country, and a punishment for all the deaths that he caused, the Almighty God grants him immortality. As he has lived for centuries, he feels lonely and depressed as he has to watch his loved ones and those around him pass away. This is until he slowly falls in love with his bride who gives him a reason to live again.

• Kim Go-eun as Ji Eun-tak[10]
  • Han Seo-jin as young Ji Eun-tak

An optimistic and bubbly 19-year-old high school student and the legendary Goblin's bride. A "Missing Soul", she can see ghosts and summon the Goblin due to her way of birth; her pregnant mom was in a hit-and-run at the time to which the Goblin intervened to save them both, before the Grim Reaper could take them. Orphaned at age 9 and pursued by the Grim Reaper, she was forced to live with her abusive aunt who was seeking her mother's life insurance money. She immediately becomes interested in the Goblin, who takes her in his home so she can pass her exams and enroll in university, while he begins to rediscover the joy of living through her. Originally oblivious to her duty of removing the sword from the Goblin's chest, she eventually learns the truth and struggles with the decision of fulfilling her duty as she has fallen in love with the Goblin.

• Lee Dong-wook as Grim Reaper / Wang Yeo[11]
  • Kim Min-jae as young Wang Yeo

A good-looking, cynical yet humorous grim reaper (Angel of Death) who guides souls to their reincarnations or afterlives and Kim Shin's housemate. Along with many other grim reapers, he became a grim reaper after committing the greatest sin of all in his past life. He has no memory of his past lives, but does possess supernatural powers. Originally preferring to be by himself, he falls in love with a chicken restaurant owner named Sunny who eventually reciprocates his feelings. Through her, he begins to unlock clues to his memories and who he was in the past. His past life was revealed to be the young king: Wang Yeo. Despite the king's original pleasant personality, he became corrupted by the power hungry eunuch and orders his guards to kill everyone including his own wife and his loyal general, Kim Shin. This act drove Wang Yeo insane and caused him to ultimately take his own life.

• Yoo In-na as Sunny / Kim Sun[12]
  • Kim So-hyun as young Kim Sun

The attractive owner of a chicken restaurant who hires Ji Eun-tak. The Grim Reaper begins to fall in love with her, and she soon returns his affection, although she is constantly confused by the Grim Reaper's constant social awkwardness. In her past life, it is revealed that she was the younger sister of General Kim Shin and she became the queen upon marrying King Wang Yeo (now the Grim Reaper). The couple were happily in love until the eunuch began to warp the mind of the king. When Wang Yeo tried to exile his loyal general Kim Shin, she supported her brother as he stood up to the king's ridiculous order and was executed as a result.

• Yook Sung-jae as Yoo Deok-hwa[12][13]
  • Jung Ji-hoon as young Yoo Deok-hwa
  • Kim Hyun-bin as teenage Deok-hwa

A rebellious yet kind-hearted chaebol heir and the only grandson of the Yoo family, a household that has the responsibility of taking care of the Goblin. He originally is a spoiled young adult who was only looking after the Goblin in order to receive a new credit card (after he was cut off). However, he learns of the Goblin and the Grim Reaper's true identities, and the trio grew to become strong friends. It is later revealed that the Almighty God had taken over his body for a period of time so that he could enact his plan of intertwining the lives of Kim Shin, Ji Eun-tak, The Grim Reaper, and Sunny

• Lee El as Samshin, Goddess, the creator of birth and fate
• Kim Byung-chul as Park Joong-heon, a cunning and manipulative eunuch from the Goryeo Dynasty
• Kim Sung-kyum as Chairman Yoo Shin-woo, Deok-hwa's grandfather
• Jo Woo-jin as Kim Do-young, Deok-hwa's secretary
• Yeom Hye-ran as Ji Yeon-sook, Eun-tak's maternal aunt
• Jung Young-gi as Park Kyung-shik, Eun-tak's male cousin
• Choi Ri as Park Kyung-mi, Eun-tak's female cousin

• Hwang Seok-jeong as triplet sisters; two of them as a shaman fortune teller and a ghost
• Ham Sung-min as North Korean soldier

• Ahn Ji-hyun as Go Jung-hyun, the ghost in the library who was Eun-tak's mother's best friend
• Park Kyung-hye as Virgin Ghost, who always tries to take Eun-tak with her
• Park Se-wan as Go Shi-won, the ghost who asked Eun-tak to clean her dormitory and fill it up with food so that her mother will not be sad (Ep. 2, 4-5)
• Kim So-ra as Lee Jung-hwa, a vengeance ghost whose husband killed her (Ep. 4, 6-7, 9, 11)
• Go Soo-jung as a ghost (Ep. 2)

• Noh Gang-min as the boy in the back-alley of the chicken shop who thinks he has wind powers
• Kim Min-young as Park Soo-jin, Eun-tak's schoolmate who harasses her and takes pictures of her and the Goblin
• Go Bo-gyeol as Kim Yoon-ah, Eun-tak's class president who defends her and later becomes her friend
• Kim Nan-hee as Eun-tak's homeroom teacher who discriminates against her because she's an orphan
• Ma Min-hee as one of Soo-jin's friends
• Yoon Hee-kyung as one of Soo-jin's friends

• Kim Ki-doo as a Batch 21 Grim Reaper who is very serious about his work
• Choi Woong as Wang Yeo's junior grim reaper who lives in the rooftop above Sunny's apartment
• Jo Hyun-sik as a Batch 22 Grim Reaper
• Kim Chang-hwan as a Batch 23 Grim Reaper
• Yun Da-yeong as a Batch 23 Grim Reaper who was once Wang Yeo's Court Lady

• Kim Nam-hee as Overworked doctor in ER
• Yoon Kyung-ho as Kim Woo-sik, Kim Shin's loyal lieutenant from the Goryeo era (Ep. 1, 12, 14)
• Lee Moon-soo as a loyal old man from the Goryeo era who kept Kim Shin's body (Ep. 1)
• Nam Da-reum as Kim Soo-bok, the boy who met the Goblin in Paris (Ep. 1, 4)
• Park Jin Woo as Loan Shark (Ep. 2-3, 6)
• Yoon Joo-man as Loan Shark (Ep. 2-3, 6)
• Lee Seul-bi as a vain woman at café (Ep. 3)
• Kim Hyun-mok as Bus Passenger (Ep. 8)
• Kim Hye-yoon as the young era of a widowed old lady (Ep. 15)

• Park Hee-von as Ji Yeon-hee, Ji Eun-tak's mother (Ep. 1, 7)
• Kim Min-jae as Wang Yeo (Ep. 1, 7-13)[14]
• Kim So-hyun as Kim Shin's sister (Queen)
• Jung Hae-in as Choi Tae-hee, Ji Eun-tak's first love (Ep. 7, 8)[15]

The series was written by Kim Eun-sook, who also wrote the popular series Secret Garden (2010), The Heirs (2013), and Descendants of the Sun (2016). The drama marked her second collaboration with director Lee Eun-bok after both worked on Descendants.[16][17]

The first script-reading was held at Nuri Dream Square in Sangam-dong, Seoul, South Korea, on 30 August 2016.[18] The Goryeo-era flashback scenes were filmed on Gimje, North Jeolla Province on September 22[19] for the battlefield scenes, while the palace scenes were shot at Naju Image Theme Park.[20] Goblin and Grim Reaper's house (external shots) was filmed at (formerly) Unhyeongung's Western House,[21] presently inside Duksung Women's University (utilized for administration offices).[22]

Overseas filming mainly took place in Quebec City, Quebec, Canada in October,[23] using locations such as the Château Frontenac, Parc du Bastion-de-la-Reine (the Yoo family cemetery), Petit Champlain (Goblin's door to Canada), and the Fontaine de Tourny.[24] The red door, which in reality is the emergency exit of the Théâtre Petit Champlain, and other sites in Quebec City associated with the show have begun to attract numerous fans of the series.[25]

The crew was awarded a special vacation to Phuket, Thailand after the end of the drama.[26]

The series airs on tvN, a cable channel/pay TV which normally has a relatively smaller audience compared to free-to-air TV/public broadcasters (KBS, SBS, MBC and EBS). The drama consistently topped cable television viewership ratings in its time slot. Its final episode recorded an 18.680% nationwide audience share according to Nielsen paid platform, making the episode the second highest rated in Korean cable television history.[7] It became the first cable drama to surpass 20% ratings.[65]