Plo Koon is a Kel Dor Jedi Master from Dorin, a member of the High Council across all three prequel films, played by Alan Ruscoe in The Phantom Menace (1999) and voiced by Matt Sloan in The Clone Wars. The mask and goggles are not costume flourishes: Kel Dor physiology cannot handle an oxygen-rich atmosphere, and his own homeworld's air is what the breath mask supplies.
On film he has almost nothing to do. The 2008 animated series rebuilt him from the mask outwards and made him one of its most quietly effective figures — a Jedi general who treats clone troopers as people with names rather than as materiel, and who is given the Order's most humane instincts without ever being written as soft.
He also became, in canon, the Jedi who found Ahsoka Tano on Shili and brought her to the Temple, which quietly makes him the reason the Order's most-discussed padawan exists at all. His death is one of the most economical in Star Wars: his own escort fighters open fire on his cockpit the moment Order 66 is transmitted, and the series does not linger.
Nothing in his publishing history is essential reading, which is itself worth saying on a reference site. Plo Koon is a television character with a film credit, and the place to meet him is The Clone Wars.