Before we "Meet Kevin Johnson" tonight, I thought it would be helpful to revisit what we know about Michael and Walt. [Keep in mind that I am on the West Coast and it hasn't aired yet, so if you've already watched it, please keep comments spoiler-free until tomorrow. Thank you!]
Michael Dawson worked in construction (presumably for Widmore Construction) and was an artist as well. He used to live with his girlfriend Susan, who is Walt's mother. Shortly after Walt was born, they broke up. Susan, who was a lawyer, took a job abroad and brought Walt with her, denying Michael the chance to be his father.
After an argument with her on a pay phone, Michael was hit by a car in NYC. Susan paid his medical expenses, because she wanted him to hand over all parental rights to her and her new husband (Brian). Several legal battles ensued, but Michael agreed in the end that Walt would have a good life with them.
Susan then passed away from a rare blood disease, and Michael went to Australia to get Walt because Brian was concerned about his odd behavior. Michael and Walt then hopped on Flight 815 together.
Michael and Walt's relationship has never been stellar, but they slowly bonded after they crashed on the island. When Michael built a raft to try and find rescue, Walt burned it down because "I like it here." Michael built another raft, and set sail with Walt, Sawyer and Jin. They encountered a boat that night, and the Others stole Walt off of the raft before blowing it up.
Michael, Sawyer and Jin were shipwrecked but drifted back to the island. Shortly after they reached the shore, they were taken hostage by Flight 815 Tail Section survivors Mr. Eko and Ana Lucia, whom they thought were Others. Eventually they made peace and ventured back to the beach to reunite with Jack and the other survivors.
Meanwhile, in Otherville, Juliet and Ben devised a plan to use Walt as bait to lure Michael into doing them a favor (bringing Jack, Kate and Sawyer). While in the original Swan hatch, Michael received an instant message from Walt, giving him directions to find him. Michael knew he had to go alone so he tricked and locked up Jack and Locke, took a gun and left to search for Walt. Shortly thereafter, he was taken by Tom and the Others.
During this time, Ben arranged for his own capture by Jack and the survivors. They kept him in the Swan hatch while trying to figure out if he was an Other.
Michael was taken to a fake Others camp and they took a sample of his blood. Ms. Klugh interrogated Michael about Walt, and he was their prisoner for a week before they brought Walt for a brief visit. Then Klugh explained the deal to Michael: if Michael released Ben from the Swan and brought Jack, Kate, Sawyer and Hurley to the Others, he would get Walt back. Michael bartered for a boat as well, and his request was granted. Then Juliet visited him and told him that Walt was special.
After Michael was released, he encountered Jack and Kate in the jungle and they brought him back to the Swan. He lied and told them that he hadn't been taken, but that he knew where to find the Others. At one point, he was alone in the hatch with Ana Lucia, who was guarding Ben. Out of the blue, Michael shot and killed her, as well as Libby, who had wandered in at the wrong time. He then let Ben go, and shot himself in the arm to make it look like Ben shot him.
Michael organized a rescue mission, but insisted that only Jack, Sawyer, Kate and Hurley went along (as instructed by the Others). Sayid became suspicious of this and alerted Jack. While on the trek to find Walt, Jack forced Michael to admit his real plan, and then all of them were captured by the Others. Ben let Hurley go, returned Walt to Michael and gave them a boat with coordinates for rescue. The Others then took Kate, Sawyer and Jack back to Other Village. That was the Season 2 finale, and didn't see Michael until...
Michael showed up on the freighter, working on the crew and using the name Kevin Johnson.
Walt was named after Michael's grandfather, but he hadn't seen his biological father (Michael) since he was 2 years old. After Walt's mom died and his stepfather (Brian) couldn't deal with him, Michael went to Australia to reconnect and bring him home. Brian noticed that strange things were happening around Walt, including a time when he was reading about a rare bird and suddenly that very bird crashed into their window and died.
He was 10 years old when Flight 815 crashed. On the island, Walt bonded with Locke over backgammon and knives, much to Michael's chagrin. Walt also began displaying signs of clairvoyance, warning Locke not to open the hatch even though Locke didn't tell anyone about it.
When Walt left with his dad on the raft, he gave his dog Vincent to Shannon. Yeah, she died. No one in particular seems to be in charge of him now. But before she was killed, a soaking wet Walt (or his apparition) appeared to Shannon in the jungle, supposedly whispering the following in reverse: "push the button, don't push the button, bad." A few days later, Wet Walt showed up in her Sayid love tent and whispered in reverse: "they're coming, they're close." Shannon chased after him and eventually even Sayid saw him, but in the rainy jungle ghost commotion, Ana Lucia accidentally shot and killed Shannon. Oops.
After Walt was taken from Michael on the raft, the Others took him to Hydra Island and subjected him to tests in Room 23 (where Karl was later discovered). Apparently Walt demonstrated some of his powers and scared the bejesus out of them. But Ben insisted that Jacob wanted Walt to remain there because he was special.
Walt was briefly reunited with Michael at the fake Others camp, but didn't see his dad again until Michael followed up with his end of the deal by leading Jack, Kate, Sawyer and Hurley to the Others. Walt and Michael left on the boat provided by Ben, following the coordinates for rescue. Again, that was the Season 2 finale, and we didn't see Walt again until...
Walt appeared to Locke just as he was about to kill himself because Ben had shot him. Tall Ghost Walt told Locke that he had work to do, so Locke maneuvered his kidney-less, bullet-ridden self right up out of that mass grave and went back to being crazy.
So there you go.