Other forms: honester; honestest
The adjective honest is perfect for describing someone who tells the truth. If you're always honest, it means you're truthful and sincere no matter what.
Honest comes from the Latin word honestus, which means "honorable or respected," and around 1300, honest was popularly used to mean "respectable and of neat appearance." We don't use it these days to describe the way someone dresses, but instead how truthful they are, and sometimes to emphasize how simple and straightforward something is, like "good, honest home-cooked food."