truthful
You were right to speak truthfully to the police.
Truthfully, I did find it hard to settle down.
You should respond as truthfully as possible.

SMART Vocabulary: related words and phrases

Honesty, openness & sincerity
above board
anti-corruption
artless
artlessly
artlessness
earnest
honestly
in
in all honesty/seriousness/truthfulnessidiom
in good faithidiom
integrity
joking apart/asideidiom
sincerely
squeaky-clean
stainless
straight arrowidiom
straight upidiom
the straight and narrowidiom
transparency
upright