Since Then: When Shannon's father died, and Boone's mother inherited his money, Boone began supporting his step-sister. She began conning him for further money, repeatedly faking bad relationships so Boone would pay off the abusive boyfriends. At some point, Boone fell in love with Shannon, and she seduced him the night before Oceanic 815's flight. On the island, the two bickered lightly, and Boone tried to protect her from a relationship with Sayid. Soon before he died, Boone overcame his obsession with her. In the flash sideways, he helped reunite the two to awaken them.
Since Then: Shannon has had several short-term relationships and all of them seemingly with a particular agenda. While living in Australia, Shannon and her boyfriend at the time, Bryan pull a con together on Boone by feigning an abusive relationship that Shannon knew Boone would try to intervene on her behalf. Sure enough, Boone offers Bryan $50,000 to leave Shannon and tells Bryan that he (Bryan) would be the third of Shannon's boyfriends he would attempt to pay off. Shortly thereafter, Bryan leaves Shannon and takes the money with him.
Origin: Locke used Boone as an apprentice, and Shannon was very suspicious of what they were doing. ("Hearts and Minds"). Later, she believed that he killed her step brother Boone("Deus Ex Machina").
Prize: Revenge. Shannon believed Locke was responsible for her brother's death and wanted revenge.
Fuel: Locke and Boone found The Hatch in the jungle. ("Whatever the Case May Be"). Shannon became increasingly suspicious about what they were doing. Boone's death further complicated matters. Shannon stole the key to the case of guns and nearly killed Locke--but Sayid stopped her from doing so ("The Greater Good").
Resolved: Maybe. Shannon's death has seemed to resolved this rivalry, as Locke was seen at her funeral dropping sand on her grave ("What Kate Did").
Origin: Shannon and Sayid began their bond when Sayid asked Shannon to help him translate Rousseau's French transmissions.
Since Then: Over time, the two became close. Boone's death became a test for their relationship as Shannon turned inward, and asked Sayid to kill Locke for her in revenge for her belief that Locke killed her brother. When Sayid refused, Shannon took it upon herself to do the deed but was stopped by Sayid in the nick of time. Despite their cultural and social differences, Sayid and Shannon eventually got back together and that culminated in a romantic night Sayid had planned out for her. However, when Shannon started seeing visions of Walt, she was upset that Sayid had trouble believing her. When Sayid saw the vision himself however, he told her he believed her and they told each other that they loved each other. However, as Shannon tried in vain to chase after Walt again, she was shot in the stomach by Ana Lucia, who thought she was an Other. Sayid buried Shannon, and spoke at her funeral on the beach.