An unreliable character is a character who you cannot depend upon to tell you everything they need to tell you. The word “unreliable” in writing usually refers to a narrator, but this type of character doesn’t alway need to be a narrator. Like almost all other character types they can be anyone which makes it more difficult to know which character is unreliable with information or helping the protagonist and which one isn’t.
The character can be unreliable in a variety of ways. Maybe they are a chronic liar, or maybe they have an addiction which causes them to forget things frequently. These two example have to do with the character being unreliable with information, but they can also be unhelpful or even an impediment to the protagonists journey. The protagonist cannot depend on them for anything.
And you cannot forget that there should always be a reason as to why these characters are so unreliable. You have to be careful here if you want to give a good exclamation because sometimes the reasons for a character can go deep into their psychology. It is possible that they have a psychological condition which makes them this way so you need to make sure that you are respectful about these conditions throughout. In order to be respectful in these situations research.