"Unperfidious" is a rare word that describes someone who is loyal and trustworthy, and not likely to betray others. Although there are not many synonyms for this word, there are several related terms that can be used to convey similar meanings. Words like faithful, dependable, devoted, and steadfast all suggest qualities of trustworthiness and reliability. Similarly, words like honest, sincere, and ethical convey a sense of integrity and uprightness. While "unperfidious" may not be a commonly used word, the concept of loyalty and trustworthiness is certainly valued, and there are many words that can be used to express it.