A true friend is an honest friend. That's the most important characteristic to me among others like honesty and sincerity. If a friend gossips about you with someone behind your back, obviously that's not a friend at all. Also, a true friend should support you and be happy about your achievements. Sometimes they should let you choose you and not them. For example, when your friend wants to move abroad, it's hurtful and you're gonna miss them for sure, but you have to let them go if that's what makes them happy.