Tom

Tom was a powerful pufferfish-type Fish-Man, known as the most skilled shipwright in the world. He was also the teacher of Iceburg and Franky.

Crown of the King
Before his life in Water 7, he lived in Fishman Island. At some point, he and Kororo left there and moved to Water 7. Tom was later responsible for building the Oro Jackson, Gold Roger's ship.

The Trial Begins
After the death of Roger, he was arrested. Although building a ship for pirates is generally not considered a crime (as the shipwrights have no control over who mans their ships), a special case had to be made for Tom because his client was the Pirate King.

To escape what would be a cruel and unnecessary fate, Tom made an agreement with the judge: given 10 years, he could save his dying home city of Water 7, by constructing an unprecedented Sea Train which could traverse even the roughest of weather to deliver and receive goods and services. Thus, Tom's trial went into recess for a decade, to reconvene after the Sea Train's completion.

The Shipbuilders Secret
Tom's trial was also a upfront for another mission the World Government was undertaking at the time. They knew Tom was the holder of a copy of the blueprints for Pluton, an ancient warship of power so great that its name survived the passage of time (or perhaps the cataclysm that blotted out most of the world's history. This is the same Pluton that Crocodile was seeking in Alabasta.

Tom's possession of the blueprint led other factions of the Government to seek it: of note, a man named Spandam, who was at the time working for Cipher Pol No. 5, a secret governmental organization not overseen by the Marine.

Spandam Moves
After Tom finally completed the Sea Train (taking 4 extra years to add extra rails), Tom was finally allowed to be pardoned for any crime he desired. However, Spandam was ever anger at the hassles of trying to get Tom to hand over Pluton to him. Spandam, in an effort to secure the blueprints, hired people to steal and man the ships of Tom's student, Cutty Flam, to attack Water Seven at the resumption of Tom's trial.

Though he knew Franky to be innocent, Tom took responsibility for the actions of his pupil's ships and decided to claim his pardon against the attack on Water 7. This action thus reinstating his original sentence and he once again was accused of building Roger's ship. Having overheard Franky disown his creatations, Tom finally owned up to building the Oro Jackson, not only that but claimed he was proud to have done so in order to show Franky the true meaning of being a Shipwright. Tom was escorted on the sea train he built to Enies Lobby, where he was allegedly executed shortly thereafter.

Trivia

 * In a recent Japanese Fan Poll, Tom is currently ranked the 41st most popular character in One Piece.
 * Like some of the other characters of One Piece, Tom had a unique laugh, he started it with a "Ta" (i.e. ta ha ha ha ha!)