These are the relationships that Duke encounters during the history of his life. They are his romantic, business, friendly, and enemy relationships.
Duke had a strained relationship with his father Simon Crocker. Simon made Duke promise to come back to Haven should The Troubles ever return. Before his death, Simon left Duke the Cape Rouge, where he arranged for it to come into Duke's hands in a poker game. He also left Duke the Crocker box, which had their family history and the truth behind their lineage.
Duke and Julia grew up in Haven together. Duke claims when Julia was little she used to follow him around due to her "huge" crush on him. They reunite years later in As You Were when Julia returns to Haven. Duke is there when her mother dies and he comforts her. In The Trial of Audrey Parker, Julia joins in on Duke's poker game he was hosting on his boat when Ezra Colbert and Tobias Blaine takes them all hostage for a box that Duke has hidden on his boat. Duke tries to protect Julia the best he can, but they are separated and Duke is tortured for the box location. It is when Julia screams that Duke thinks of the location and Ezra catches that thought. After the incident, Julia is disgusted by Duke's lifestyle and tells him to stay away from her. In Spiral, Duke meets Julia outside Audrey's room, and she is still angry with him. Julia sends him away, but Duke asks Julia to tell Audrey about Max Hansen. Hearing about Duke looking for a man with a tattoo, Julia shows him a picture of her family with men who have the tattoo. Julia takes Duke to a graveyard filled with gravestones with the tattoo symbol on it, showing Duke there are many people with it. Later on Duke writes Julia's name on a blackboard of suspects.A Tale of Two Audreys, Duke was surprised to see her back and knows Evi is up to something. Evi is surprised that Duke is still in Haven since he never liked the town. In Love Machine, Evi thinks Duke is working an angle with the other Audrey and thinks the file in the car is something the FBI agent has on him and steals it for Duke. She later brings Duke the file asking what it is, and Duke shoos her away. In Sparks and Recreation, Evi is also looking for what Duke is searching for. She forces him to work with her on the case.
Jennifer and Duke first met in Boston after the Barn spat Duke out. Jennifer helped Duke escape the hospital and meet up with Nathan. After returning to Haven, Jennifer and Duke form a congenial relationship, with him acting as her landlord and guide to Haven and the Troubled. Audrey's appearance puts a strain on their relationship, with Jennifer believing Duke wants Audrey who is pretending to be Lexie to fill Audrey's place--the role of closest female friend-- in Duke's life.
When Duke is possessed by Tyler, a Troubled young man in The New Girl, he kisses Jennifer. Though she was surprised, her reaction is positive, and she agrees to let "Puppet Duke" accompany her when she moves back to Boston. When Puppet Duke stops by Haven's medical center, he behaves in ways that let Jennifer know he's been possessed, and she threatens him with a gun.
When Duke is no longer possessed by Tyler, he offers Jennifer a room on his boat. They talk about the kiss between Puppet Duke and Jennifer, and Duke points out that Jennifer kissed back.
After killing and burying his brother, Duke reexamines his life and decides he's become a "schmuck". He asks Jennifer to remove her things from his boat and seriously considers leaving Haven, but when two of Reverend Driscoll's nephews develop a Trouble and Audrey is able to solve it with Duke's help but without his Trouble, Duke decides to stay in Haven. After making this decision, Duke instigates a physical relationship and Jennifer reciprocates.
Wade was Duke's older half-brother who used to live in Haven until his mother took Wade to the city. Wade was present during the incident when Nathan's trouble kicked in. They grew up apart. When Duke disappears for 6 months, Wade returns to Haven to take care of Duke's affairs and of the Grey Gull. After he learns about Crocker family curse he wants to kill Jennifer but Duke stop him by killing him instead.
Duke and Dwight have a complicated friendship.
Duke and Mara have a complicated relationship. At first, Duke hated Mara since she was not Audrey and at first was going after Jennifer. He continued on disliking her even when Nathan was trying to find Audrey inside of Mara, and when the guard was trying to get Mara to cure The Troubles. During this time, Mara helps Duke to release a trouble in order to save his life for awhile. In The Old Switcheroo: Part 1, Duke goes along with Nathan's plan of treating Mara as Audrey to save Audrey. In order to manipulate Duke, Mara pretends to Audrey and uses the memory of Colorado to get Duke to free her. Just as Duke figures out Mara is playing him, he switches bodies with Nathan. Duke seems to be attracted to Mara, but until Reflections we don't know if Mara feels the same. In Reflections Duke and Mara share first kiss and sleep together.