If you've been watching "Castaway Diva" and liked the cute brotherhood relationship in it, here are some more K-Dramas with a lot of focus on strong and protective relationships between brothers that may or may not be biologically related.

Under the Queen's Umbrella

This drama mainly focuses on motherhood and how mothers go to great lengths for their child (in this case, even if they're not her child!), but the princes also showcased a great amount of brotherhood that is just so heartwarming to watch. 

It's Okay to Not Be Okay

This emotional drama is popular for their love story and the strong brotherhood relationship it portrays. Moon Gang-tae is a sweet, caring brother who takes care of his autistic older brother, Moon Sang-tae, as they frequently move from town to town ever since Sang-tae witnessed their mother's murder.

Tale of the Nine-Tailed (both seasons, but more so for 1938)

You could see this drama's strong brotherly bond in the first season, but the second season definitely focused more on that, and it was quite an endearing tale to watch, too. It reminded me of the Western show, CW's The Vampire Diaries, where brotherhood played a vital role in their story and having to balance that with romance and saving the world. 

Bloodhounds

They're not biologically related, but the two have a bond stronger than blood that they made sure they both would make it out of their bloody problems alive! 

Hello Monster / I Remember You

This 2015 thriller is a must-watch, especially with the excellent acting from Seo Inguk and Park Bogum, who were recognized and applauded when it was airing.

He is Psychometric

The male lead isn't the brightest, but he is exceptionally handsome and does have the ability to help him take out evil people in the world; his psychometry ability (when he touches someone, he can read the person's memory). 

Oasis

This one's underrated from this year, but it's certainly a good one to add to your watchlist if you haven't! Set against the turbulent backdrop of South Korea from the 1980s to the 1990s, this drama tells the story of three young people who fight fiercely to protect their dreams, friendship, and their one and only first love!

My Mister

This one has to be a classic and a must-watch! This critically acclaimed drama is about a man in his 40s withstanding the weight of life with his two brothers, while a woman in her 20s who is a temp at his workplace also goes through difficult experiences. The man and woman get together to help each other.

Circle: Two Worlds Connected

It's another underrated one, but you'll thank me for this one! Set in both the present day and far future of 2037, an alien comes to Earth, believing that if human emotions are not controlled, they will have no future.

Films:
Inseparable Bros

The two male leads are not blood-related brothers, but they have been like brothers for the past 20 years. One is smart, but he has a physical disability, while the other is not very smart, but he is in excellent physical condition. What happens when one of the brother's mother suddenly visits and acts as his guardian? 

My Annoying Brother

This comedy film will have you laughing and maybe... even relate to your annoying sibling(s). Watch how these two estranged brothers have to live together despite hating each other.