He has a serious demeanor and a strong sense of duty.

Holding a bottle in each hand, she turned around and assumed a serious demeanor.

She shot me a quick glance, but I maintained a perfectly serious demeanor.

He is a short man with close-cropped hair and a serious demeanor.

He stopped abruptly, though, and assumed a more serious demeanor.

This is offset somewhat by a mature and serious demeanor.

She took on an extremely serious demeanor as well, and turned to Joseph.

Her mother's face drifted to a more serious demeanor.

Her hair is longer, and she has a more serious demeanor.

Dimitri kept a serious demeanor though he wanted to grin.