In addition to what Yuliya says, you can describe someone: as "not afraid to voice their opinion", that "says what is on their mind", that "does not sugar coat the truth". "unfiltered" or "has no filter" is another way to describe someone, but this usually has a negative connotation. another related adjective is "opinionated". hope this helps

You could say "He's very direct" Here's an another example from Cambridge Dictionary "(of a person) honest and not likely to hide their opinions: Roz is straightforward and lets you know what she's thinking."

Which the best one word?