"easygoing demeanor" is correct and can be used in written English. It is used to describe someone who has a relaxed, laid-back attitude towards life. For example, you can say, "He had an easygoing demeanor and never seemed to be stressed or anxious.".
He was a natural card player with an easygoing demeanor.
Geddes's optimistic and easygoing demeanor smoothes Fernandez's jagged edges.
Els's easygoing demeanor should not be mistaken for a lack of pride, however.
Camby, whose easygoing demeanor off the court is sometimes mistaken for weakness on it, says he will be "judicious" principal.
"His demeanor, easygoing attitude, a sports nut — I'm kind of the same mold," Henry said.
Leyritz's easygoing, charismatic demeanor made him popular on the speaking circuit.
He had a gray beard and longish gray hair, and an easygoing, jokey demeanor despite his recent troubles.
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Simone Ivan Conte
Software Engineer at Adobe, UK