User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-39.43.215.69-20180421040053/@comment-28884330-20180421175007

Issei became Asia's first friend when everyone told him not to, Issei risked his life to save Rias from a marriage she didn't want (add "risking his life" to pretty much EVERYTHING notable he has done), decided to help Yuuto, the "damn handsome" according to him, even against the wishes of Rias (shows a lot of his character that he's helping someone who is not a beautiful girl, which proves he helps because he's a good person, and not some ulterior motives), he restored the consciousness of Sairaorg's mother and he even saved the Underworld from the Jabberwocky (this thing alone is a much more impressive and admirable feat than what any of the guys you mentioned have done).

This is not even half of Issei's admirable actions, and his admirable actions are complemented by him being a good person regardless of them, he's admirable as a person too. He has done MUCH more great things than any of the guys you mentioned, especially Vali. And even more, Issei for a lot of the story has been weaker than all of the guys you mentioned, and he still pulled off feats that are much more admirable. He was not as strong as them, many times he has been weaker than the opponent and understandably scared when risking his life, but he has always fought on. That's the definition of "brave".