Sincere and honest in character or behavior; having integrity and dealing with matters in good faith (a na-adjective).
彼はいつも誠実に仕事をこなしている。
He always does his work sincerely and reliably.
誠実な対応が会社の評判を高めた。
Sincere handling improved the company's reputation.
誠実さは信頼関係を築くうえで重要だ。
Honesty is important for building trust.