"Dobby?" he asked. Jack’s vision had blurred on account of all the crying he’d been doing and so he couldnt clearly distinguish the face of his captor. From what he did see however, he believed the man before him was actually the famous house elf from The Harry Potter books. Dobby was Jack’s favorite character so seeing him was actually sort of a relief.
But as his vision cleared up, he realized it was not Dobby, but the man they knew as Henry Gale.

“When we get out of this thing, Jackass, I’m gonna kill you,” Sawyer said through gritted teeth.
All Jack would say was, “I’ve got a plan.”
[...]
“We were here when Boone died!” “Niki” whined. “We were at the raft launching. Michael almost shot me in the hatch after he got Libby. We helped put out that forest fire you started, Kate. And don’t you remember we were taken by the Others with you once?” She asked. “Paolo and I were put in cages right beside yours.... we saw you two.....”