Regarding how to gauge the reliability of statements about characters.
The answer depends on the person saying it.
Let's make a hypothetical example.
"The Green Knight is stated in-universe to be invincible. So far, he has taken every single attack thrown at him on the chin and either picked himself off the ground or absolutely tanked it, coming out completely unharmed either way. However, he lives in a low fantasy setting. Is he truly invincible?"
- Option 1: The person who made the claim was, say, a squire who was telling the heroes about how he slew his master. We would consider this source hyperbole and an example of the Hasty Generalization fallacy (which, for the record, is the father of the No-Limits fallacy). He saw him break a lance on his forehead and deemed him indestructible.
- Option 2: The person is an educated wizard who has been researching this character and experimenting on him with his most destructive magic. In this case, the source is more reliable, but still not completely believable. He is stated to be invincible. In truth, he is only invincible in comparison to whatever this wizard could dish out. A low fantasy "indestructible" is our city-block level.
- Option 3: A 2-C or higher deity says the Green Knight is indestructible. If someone with infinite power and the ability to tear this person's reality apart says they can't kill this character, there's an indestructible. At least against all conventional forms of damage. He might be defeated by a polymorph spell or something else that ruins durability.
- Option 4: The narrative itself calls the Green Knight indestructible. This statement could be interpreted similarly to Option 2 in that it should be examined in the context of the story. For example, an indestructible character in a low fantasy setting would not automatically be indestructible in settings with more destructive weapons or powers.
- Option 5: The author is the one calling the Green Knight indestructible outside of the story. As the previous option, this should be examined in the context of the story. A character being described as invincible, indestructible or all powerful in one setting might not be in the same position when compared to characters from other settings.
A character CAN be upgraded by word of mouth alone. You just have to double check to see by how much, as well as:
- If the source of the statement is reliable?
- If the statement conflicts with the story or feats of the character?
- If the statement is only valid in the context of its setting, or if it holds up in comparison with other settings?