I feel like the addition of Cynthia (as the given name) would be supplemental to the already given translation of "sincere". It's very rare for a Japanese person to use sincere as its katakana derivative. Additionally, Cynthia is commonly used as the katakana name for Saori Minami during her performance years in the 1970s as a kayokyoku artist.
Cynthia vs. Sincere
I feel like the addition of Cynthia (as the given name) would be supplemental to the already given translation of "sincere". It's very rare for a Japanese person to use sincere as its katakana derivative. Additionally, Cynthia is commonly used as the katakana name for Saori Minami during her performance years in the 1970s as a kayokyoku artist.