I think many of the champions are scared and uncertain. For example, we have 
.
Amumu is constantly unsure and sad as a champion of course. He is cursed to forever to be alone and probably doesn't know why he keeps searching for a friend at times. His story is simple, yet likely the saddest in League. He is depression incarnate, and probably has no real self-confidence at all.
Galio is unsure of what he should do most of the time. Sure, on the outside, he appears to be some big oaf that loves to constantly fight, and he probably is. However, if he kills whatever is causing him to be alive... well, he's no longer alive afterwards. While he would probably never put himself over others, there's probably a small part of him that is unsure if killing every last mage would be the best for himself. Is he doing what is right? When I kill a mage, should I really be killing them?
Lux is unsure of her place in her kingdom and even family. In a nation that taboos magic of most kinds, Lux doesn't really know if what she does is right and where it might lead her in life. In her lore she has come to accept it enough to not want to get rid of it, but she still doesn't know what her powers will do for her in life.
Nami, of course, has the weight of her tribe put upon her. She isn't the oldest and is a little naive, making the weight that much more on her. She's not sure what is right and wrong, or what what is so to speak. She's exploring a world completely foreign to her, making her of course have doubts on if she can support her tribe and perform her duty.
Poppy... Poppy's lack of self-confidence is part of her character. While it could be described as simple naivety and ignorance, Poppy doesn't believe she herself can be a hero despite all signs pointing to it. Poppy has never stopped and believed she herself is worthy of being the hero, simply seeing herself as just the "delivery girl" so to speak for the real hero.
Taliyah has little knowledge of her own powers and isn't really sure if she can control her powers all that well. While she masters it a good bit in her lore, it's still likely that she is far from being where she needs to be. She constantly struggles to feel like she is living up to what she could be and if her powers are really for the best.
Yorick is never hopeful. While he fights on for what he believes is right, Yorick is no optimist. He knows that he might not be able to complete his quest and there is a very high chance he won't. He'll keeping fighting until that day comes, but Yorick has no doubt that he will be tested during his journey and there may be some trials he simply will fail to. The only reason Yorick hasn't given up yet is because he knows that even if he gives up he won't pass on to a peaceful afterlife. It's either carry on or be tortured for eternity, which even the most doubtful person would not want, especially since they themselves had nothing to do with the Black Mist spreading. If Yorick had caused the Black Mist, I could easily see him letting go and offing himself because he feels his past crimes are worthy of a death sentence. However, Yorick didn't cause the Black Mist. He's simply a survivor that HAS to press on. Yorick has no faith in himself, but there is no other choice for him but to press on and continue to fight, even if he wanted to quit.
Plus, I would like to note that while this game obviously has it's share of "doubtful fighters", it's called League of Legends. While a hand full can have some doubt in themselves, you don't typically become a legend by having such a ginormous fault. You become a legend by being someone big and/or doing something big, and that usually involves having to have courage and the ability to believe in yourself. While everyone has faults, the inability to believe in one's self is typically the direct opposite of what a legend of any kind, hero or villain, is.
(Really listen to his lines) 
(More lore-wise than in-game) 
's case are supernatural, dude. So it wouldn't be out of wack if non humanoid champions are missing the confidence factor.
could kill dragon/half dragons
, giant alligator demigod like champions
, or floating tentacle death monsters
without it? I think not, they would run for the hills.
is only confident in his beauty
