Lieutenant Wes Janson of the X-Wing Series is a classic example, between his pranks, general irreverence towards everything in life, and his boundless sense of humor. He is capable of being serious, when actively shooting at things, but it's not his natural state. However, he explains that this is a deliberate part of his philosophy of living life to its fullest, given the mortality rate of his chosen profession.
Janson: I want you to remember something very important: you can't look dignified when you're having fun.
Myn Donos: I... I'm asking for career advice, from a nine-year-old.