also: げんきもん
noun
lively person; energetic person; live wire
Describes a person who is always cheerful, active, and full of energy. Often used affectionately.
彼はクラスの元気者だ。
He is the lively one in the class.
うちの祖母は九十歳だけど、まだまだ元気者です。
My grandmother is 90, but she's still full of energy.
元気 is an adjective/noun meaning 'energetic' or 'in good health', while 元気者 specifically refers to a person who embodies that energy.
Compound of 元気 (genki, 'energy, vitality') and 者 (mono, 'person').