Aichi Sendou

Aichi Sendou is the main protagonist of the Cardfight Vanguard anime and manga.

He is a somewhat shy and timid young man who wishes to be a Vanguard Fighter, but has never had the chance to have a real fight until his second encounter with Kai. When starting to play Vanguard, he believes more in himself, becoming a powerful Vanguard Fighter. His Vanguard Circle's color is blue; but when using a Shadow Paladin deck, the color changes to dark blue.

(Source: Cardfight!! Vanguard Wiki)