Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
fondly [ˈfɔndlɪ] adv (lovingly) ла́сково; (naïvely) наи́вно
he fondly believed that ... он наи́вно ве́рил, что ...
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.