Only leading roles’ character description has been provided in MBC’s Triangle Official Webpage as of now.
Jang Dong Cheol (Huh Young Dal) / Kim Jaejoong
You have to be living under a rock not to have heard of Huh Young Dal in Sabuk, Jeong Seon. Even outsiders visiting Jeong Seon Casino would soon get to know his name. His main claim to notoriety goes to his trashy life, like a scumbag. The thing he mainly does is collecting debts, helping with casino loan sharks. He doesn’t have an ounce of shame nor does he have a thing to bring disgrace on himself. Even scamps don’t like to be put on the same level as him. At the age of 10, his big brother leaves the orphanage where they lived together, saying “I’ll be back soon.” Despite trying to prevent his baby brother Dong Joon from being adopted, he fails and runs away from the orphanage. After that, he gains a new name Huh Young Dal, drifting in and out of Youth Detention Center. He comes back to Sabuk after turning twenty years old and he does everything for a living by any means necessary. But there is a girl whom even a shameless Dong Cheol feels ashamed in front of. She is Oh Jeong Hee, a casino dealer whom Dong Cheol falls in love at first sight. For him, she is someone like the fruit of the Tree of Knowledge that makes him feel ashamed of his miserableness for the first time in his life. He feels suffocated by just watching her from afar. Meanwhile, Jang Dong Soo shows up as a detective investigating the case of Goh Bok Tae who is to be his boss.
Jang Dong Soo / Lee Beom Soo
As a detective for 10 years, he handled a number of cases and renders distinguished services. But he hasn’t been treated as such nor promoted. It’s all because of his anger control disorder that makes numerous incidents. Only Hwang Shin Hye, a psychiatrist-turned-forensic-profiler knows of his psychiatric problem in police department. Dong Soo had two younger brothers. Two year younger Dong Cheol and a baby Dong Joon. They were called ‘ugly three brothers’ in the town. Their father died when Dong Soo was merely twelve years old, and later they were deserted by their mother who was economically distressed, which made the three brothers become orphans overnight. While having a tough life in the orphanage, he loses Dong Cheol and Dong Joon. Though 30 years passed after that, he can’t still trace his two brothers’ whereabouts. Shin Hye supposes that his self-condemnation about the loss of his brothers and resentment towards the world that made him undergo a tough childhood caused him a psychiatric problem. Now… Dong Soo heads for Sabuk searching for clues to the consecutive murder incident, meets Huh Young Dal who is living like a scumbag, and finds out Young Dal is Dong Soo’s lost brother Dong Cheol.
Jang Dong Joon (Yoon Yang Ha) / Yim Si Wan
Youngest brother of Jang Dong Soo. He as a newborn baby is adopted to Yoon Tae Joon’s , a rich family who has run a mining company from Grandfather. Thanks to that, Dong Joon grows up in comfort and well cared for. During his adolescence, a stormy period, he happens to know that he is an adopted child and was adrift. It doesn’t last long but changes him into a cynical and glacial person. It plants desperation in his heart that his parents could dissolve the adoption any time. The more he wants to cover his trauma, the more he gives people the cold shoulder. A lot of girls are tantalized by this stunningly handsome him with such cold personality. He never dates with anyone, even to be thought as a misogynist. The more he makes spiteful remarks about girls, the bigger love and interest they plants in their hearts. But his tightly shut heart opens after he meets a casino dealer Oh Jeong Hee. As his cold and disinterested heart opens for the first time in his life, the way he courts her can’t be quiet. Dong Joon gets to know that Jeong Hee has a gangster hovering around her and he can’t stand the fact that such guy likes her.
Hwang Shin Hye / Oh Yeon Soo
When in high school, her father Hwang Jeong Man brings an ugly boy home. He is Jang Dong Soo, known to be an orphan and a juvenile delinquent coming in and out of youth detention center. As time goes by, she becomes friends with Dong Soo and calls him ‘oppa’, but their relationship could not develop any further as he first cuts out. After graduating from a prestigious college of medicine, she marries to Oh Sang Pil but is separated later because of their troubled relationship. And she goes to United Kingdom to study and also to have a fresh start to a new life. She studied forensic profiling there, returns home and enters the National Police Agency as a profiler. She meets Dong Soo again and knows that Dong Soo is still seeing herself. She decides to finish their stifled love that they keep away from each other although they have each other in mind. However, the moment she confesses her love to Dong Joon, they meet a crisis which is fatal to their relationship.
Oh Jeong Hee / Baek Jin Hee
Daughter of a miner, born in Taebaek. Her father dies after suffering from pneumoconiosis for a long time and she becomes a head of household with four younger siblings. She gets a job in Jeong Seon Casino which gives preferences to its locals. Looking at the face of her poor life she must be in dismal, but she is always bright and optimistic. Though her studies are pushed aside in her school days, her beauty captivates many guys in her town and she also dates with some of them. She thinks being a casino dealer is the biggest leap she can make in her life and she takes herself laudable that she can pay for her siblings’ tuition. And small changes come along in her life after she meets Huh Young Dal. She feels sad with no particular reason when she sees Huh Young Dal. She never knows why her heart aches for him. She thinks she’d hold his hands if he gives his hands. But he only watches her from afar and she can’t also approach him any further. Around then, another guy Yoon Yang Ha appears in front of her. He enters as a casino marketing promotion manager and is stunningly handsome. He receives attention from all the female casino dealers, but he is interested in Jeong Hee only and she becomes the target of their jealousy. At that time, she could never expect that Huh Young Dal and Yoon Yang Ha were brothers and their relation becomes the beginning of heart wrenching love…