na-adjective
friendly; sociable; approachable; open
Describes a person who is easy to talk to, unpretentious, and readily engages with others. Often used for someone who is frank and good-natured without being overbearing.
彼はとても気さくな人で、初対面でもすぐに打ち解けられる。
He is a very friendly person, and you can quickly feel at ease with him even when meeting for the first time.
He spoke to me in such a friendly way that my nervousness melted away.
親しみやすい emphasizes ease of approach and likability, while 気さく focuses on an open, unguarded, and sociable personality.
社交的 means 'sociable' in a more outgoing, party-loving sense, whereas 気さく implies a natural, unpretentious friendliness.
The exact derivation is uncertain. The kanji 気 (spirit, mood) and さく (likely from 作, suggesting a natural, unaffected manner) combine to convey an easygoing temperament.