pure and innocent through and through; single-mindedly pure
Describes a person or attitude that is completely sincere, naive, and unwavering in its purity. Often carries a nuance of being almost too innocent or straightforward.
Her pure and single-minded personality moved the people around her.
His figure, single-mindedly pursuing his dream with pure innocence, gave courage to many people.
純真 means 'pure' or 'innocent' on its own, while 純真一途 adds the nuance of being single-minded or unwavering in that purity.
一途 alone means 'single-minded' or 'earnest', but 純真一途 specifically combines it with the idea of innocence and purity.
Compound of 純真 (pure, innocent) and 一途 (single-minded, earnest). The exact historical derivation is uncertain, but the combination is straightforward.