Origin: The two of them grew up in the DHARMA Initiative before joining the Others.
Since Then: While still in the Initiative, Ben regularly visited the Others and doing numerous errands for them. He eventually let Ethan in on the secret, and the younger boy accompanied him on his trips to the Hostiles' camp. Ethan was present when Ben kidnapped Alex. After the Purge, both of them joined the Others. Ethan presumably told Ben about Locke's mysterious appearance and disappearance via time travel. When Oceanic 815 crashed on the Island, Ben sent him to the beach camp to pose as a survivor and produce list of survivor names. After his death, Ben remembered him as a "gifted surgeon".
Origin: Ethan kidnapped Claire & Charlie while Claire was having false contractions.
Prize: Aaron. Because he was pressured to kidnap Claire and her baby for the Others, Ethan wanted Aaron badly enough to cut him out of Claire early, and Charlie wants to protect both Aaron and Claire.
Fuel: Ethan kidnapped Charlie and Claire when they were alone in the jungle ("Raised by Another"). To distract his pursuers, Ethan hanged Charlie hanged from a tree ("All the Best Cowboys Have Daddy Issues"). Ethan then took Claire to the Staff and (presumably) drugged her repeatedly, giving her sour water and injecting her pregnant belly. Eventually Claire escaped, but had no memory of anything since the crash- including the time she'd spent with Charlie. When Ethan found out that Claire was missing, he demanded that she be returned to him. He killed Scott when Claire wasn't returned before the deadline. Using Claire as bait, the Losties set a trap for Ethan, and captured him ("Homecoming").
Origin: Ethan noticed a pregnant Claire in the beach camp. According to Ethan, his own wife had died in childbirth, and Claire's impending danger affected him.
Since Then: Ethan appears to have approached Claire repeatedly at night with a needle. When Hurley began comparing survivor names against the plane manifest, Ethan realized he had little time to act and quickly kidnapped Claire, against Ben's instructions. He took Claire to the Staff medical station where he took care of her but kept her drugged. He showed her a potential nursery for Claire's baby, which delighted her. Before he could operate on Claire, delivering her child by Caesarean just as he had been, she escaped. He approached Charlie, demanding her return, and he tried to abduct her again - falling into the survivors' trap. Charlie killed him right after.
After death, Claire imagined herself pregnant again and found Ethan as her obstetrician. Ethan offered herdrugs to suppress her labor, ironically now reluctant to "stick her with needles".
Origin: Ethan approached Jack with some medicine that he'd allegedly found in the wreckage. The two talked of Claire's upcmoing labor.
Since Then: When Ethan kidnapped Claire and Charlie, Jack followed their trail with Kate. Ethan fought Jack, besting him easily and warning him not to follow further. Jack later set up a trap for Ethan, using Claire as bait. The survivors managed to ambush Ethan, and Jack this time managed to knock Ethan to the ground. He then kicked him repeatedly. Before he could interrogate the Other, Charlie shot him.
Origin: Juliet delivered Ethan, coming out of retirement as a fertility doctor.
Since Then: Years later, Ethan came to Miami with Richard to help him recruit her to the Island. He briefly lived in the same building as Juliet's sister Rachel, presumably to observe her. Once Juliet accepted the job, they traveled together to the island on the submarine. They were two of the Others' doctors and likely worked together in researching the island's pregnancy issues. When Oceanic 815 crashed on the Island, Ethan was working on the plumbing at Juliet's house.
Origin: Ethan spotted Locke right after a plane crashed on the island. Locke claimed Ben had appointed him leader and mysteriously disappeared.
Since Then: When Oceanic 815 crashed years later and Ethan posed as a passenger, he discovered Locke was one of the survivors. He befriended the man, offering experience as a tracker that he'd picked up while on the Island. When Ethan kidnapped Claire and Charlie, Locke tried to track them but ended up discovering the Hatch instead. When Claire returned, Locke argued against hunting Ethan directly, instead suggesting a defensive strategy. When that failed, he continued to resist attacking, till he learned of Jack's guns. He then assisted in the ambush against Ethan, yelling at the others to take him alive.
A deleted scene, which ABC has never made available, depicts a fight between Ethan and Locke in the rain.