情人 can mean a romantic partner (neutral) or a secret lover/affair partner (negative). Context matters. 情人节 (Valentine's Day) uses the neutral meaning.
culture
情人眼里出西施 is a famous proverb. 西施 (Xīshī) was one of the Four Beauties of ancient China, so the saying means 'in a lover's eyes, even an ordinary person looks like Xishi.'
