Hello, Katerina. How are you?! Answering your question: Frank and sincere are synonyms, which are words that have similar meanings. I'd say sincere is most common when referring to someone who is trying to tell his/her opinion (not lying) than frank. Straightforward: Someone who speaks directly about that subject. He/she doesn't go around when is trying to tell something. Trustful and honest can be synonyms: Someone who you can believe in. Also, it can be understood as someone who always tell the truth. I hope these answers help you!