Kelvin Inman

Before the Island

Employed Sayid as a torturer because he was one of the few capable of speaking Arabic
Speaks Arabic
Gave Sayid money so he could leave Iraq for good
Worked with Kate's stepfather in the Gulf War
Manipulated Sayid into torturing fellow soldiers under the pretense that they required an Arabic speaking person to translate.
Most likely worked for the Defense Intelligence Agency (DIA), as its logo is seen on the screen behind him in one scene.

On the Island

Operator of the Swan research station with his partner Radvinsky and, later, Desmond.
Rescued Desmond from his shipwrecked Elizabeth.
Enlisted Desmond as his partner to push the button in the Swan.
Continued working on the Blast Door Map after Radvinsky committed suicide in the Swan.
Planned to escape the Island by secretly repairing Elizabeth under the guise of investigating the Island.
Accidentally killed, or at least knocked out and severely wounded, by Desmond.
Upon his death, left Desmond to push the button in the Swan alone.
Wore a biohazard suit when he left the hatch to explore the island in an effort to convince Desmond it was dangerous to leave the hatch. The suit, however, was obviously a ploy, as the suit was compromised and he took it off as soon as he got out of the hatch.

Kelvin or Joe?

There were theories that Joe Inman from One of Them and Kelvin Inman from Live Together, Die Alone were separate characters (twins or clones), but it was confirmed by Carlton Cuse in a podcast that it's the same character, named Kelvin Joe Inman.

Seems likely based on photographic evidence, plus the fact that Kelvin speaks about being in the military.
Kelvin tells Desmond that he was "a spook" for 10 years
If so, released Sayid when he was captured during the Gulf War.
DIA or CIA agents do not typically use their real names for obvious reasons. It is likely that Kelvin is his real first name and he used "Joe" when dealing with enemy forces.

Cultural references

Kelvin is also the name of the main character in Stanisław Lem's book "Solaris". In this book, astronauts are sent to a planet that makes elements from their memories materialise. E.g. Kelvin's dead wife comes and visit him in the space station. This bears some similarities with the appearances of Jack's dead father in season 1 and Hurley's imaginary friend Dave in season 2. Just like planet Solaris, the island might make elements from the survivors' unconscious come (physically) true.
Kelvin is the SI's (International System of Units) unit of temperature
- One of the scientific mysteries of superconductors is that they do not have sharply defined Critical Temperatures. Typically, the transition from normal to superconducting state takes place over a range of about 5 degrees Kelvin. One critical temperature that falls in this range with a reading of about 94 Kelvin for YBa₂Cu₃O₇, and about 108 Kelvin for Bi₂Sr₂Ca₂Cu₃O₁₀.
In James P. Blaylock's novel "Lord Kelvin's Machine," Lord Kelvin invented an electromagnetic device which would negate the earth's magnetic field for a brief time in order to avoid a collision with an iron-bearing comet. The hero spends the first part of the novel trying to prevent the machine from being turned on, as the result would be cataclysmic. Also of note, the book, which takes place in an alternate Victorian era, makes use of Maxwell's Equations, a series of equations that describe gravity and magnetism. In the real world, Maxwell only came up with 4 equations. To account for all the impossible science that exists in the alternate world, Blaylock had his Maxwell come up with an additional 12 laws, bringing the total to a Swan friendly 16.

Theories

Kelvin wanted Flight 815 to crash, and drew Desmond out of the Hatch so that system failure would occur, pulling the plane down.
Kelvin is not dead.
- Unlikely seeing how there was blood on the rocks and Desmond was looking at his hand with shock.
Kelvin killed Radzinsky