Truth: You must say the truth, even if it hurts. Truth.. is better than lies in a long run, because if you lie, when truth hits, it will even be harder, and your character will be damaged
Trust: You must earn people's trust before people start believing you in a long run. You must uphold what you say, and do what is right.
Cooperation: Even with those two, you must learn to cooperate with people. If you are one-man army the whole time, then people may not give you a wide invitation to their group, since you have no experience with working with others. However, only cooperate with things that do good.
These are 3 things necessary in a character to be believable (at least to a certain degree)
hope this helps