One makes mistakes (that is life) 
"Even if everyone has forgotten, I will still remember."
For me, strength is more strength when it is not physical, but mental. A person who is physically strong can still get easily hurt. A person who is mentally strong can just shrug it off without a hint of hesitation.
The reason why I choose for this character, Michiru Kita, is because she is strong in that way. She didn't choose for her lifestyle which she used to hate so much, but yet you hear her (barely) complain. When she and her friends were in a competition were the losers would die, she never gave up unlike her friends. She continued to be optimistic even though the rivals had better weapons.
In the end of the manga, where everyone forgets about their adventures with the exception of her, the ferrymen and her guardian, you don't see her cry. Instead she decided to keep their memories with her and joke about her friends getting their memories back while being sad. That, to me, takes a lot courage.
If I continue I would probably write 10 pages long (I'll get a reaction like "too long, didn't read) so I stop here. But all I wanted to say is that she will always be a strong character in my eyes.
