A member of Team Caesar, and the brother of Yuri Usui.

The protagonist of this story, Sendo Aichi, is a timid boy in his third year of middle school. He had been living his life looking backward rather than forward, trying not to stand out. However, he had one thing that kept him going - the "Blaster Blade," a card from a card game that was given to him when he was little. That card is the reason why he begins to engage in Card Fights, something that changes his life drastically. 
The name of the card game is "Vanguard." The game takes place in a different planet called "Cray," and due to a never before seen play system, it becomes popular throughout the world. 
Aichi, immediately attracted by Vanguard, meets friends such as Tokura Misaki and Katsuragi Kamui, along with other rivals. Through friendly rivalry with them, Aichi begins to enjoy a fulfilling life. Aichi, however, has a goal: to once again battle with a Vanguard Fighter by the name of Kai Toshiki. Kai Toshiki is an aloof and cold-hearted high schooler who has outstanding abilities in the world of Vanguard. He is also the reason why Aichi started playing the game. For him, Toshiki is the person who saved him from his boring life and introduced him to Vanguard. In order to get better at Vanguard, Aichi puts his soul into it every day. He wishes that someday, he'll be able to battle Toshiki and have him recognize his worth. 
(Source: Crunchyroll)

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A few months have passed since the VF Circuit, and Aichi is now in High School. However Aichi is in a different high school than most of his friends, a high school where the instructors focus on looking towards the future. One day Aichi admits he thinks Cardfight can be a future people can believe in, but in order to prove it Aichi must use his new deck, a deck in which Royal Paladins and Gold Paladins are combined as one force. Slowly but surely Aichi must gain friends through Cardfighting and help his new team win the newly formed high school cardfighting championships. 
(Source: Anime News Network)

Aichi and his friends have won the National Tournament and decide to go their own ways until a mysterious boy named Takuto Tatsunagi appears. He reveals that the Royal Paladins, Shadow Paladins, and Kagero leaders have been captured, and the only way to free Cray is for the Vanguard fighters to use a new power- Limit Break. Because the Royal Paladins lose the ability to fight, Aichi's deck is changed to be a Gold Paladin deck. Aichi and his friends must now win the Vanguard VF Circuit if they are to learn more about the occurrences on Cray and free their old friends. 
(Source: Anime News Network)

A new generation of Cardfighting is about to begin. Chrono Shindou is your average middle schooler. His looks make people afraid of him, but he has a kind heart and is loved by children. One day Chrono opens his locker and finds a Cardfight deck no one has ever heard of: Gear Chronicle. Chrono takes it to the nearby Card Capital 2 where he meets one Kamui Katsuragi. Kamui teaches Chrono how to play the game and opens the way for Chrono to gain new friends and rivals solely by having fun, including soon to be teammates Shion Kiba and Tokoha Anjou. (Source: Anime News Network)

Several days after the mortal battle against Link Joker which embroils the whole world, the world has returned to peace. 
But Kai Toshiki faces a surprising situation. Sendou Aichi, the hero who saved the world from the invasion of Link Joker, has suddenly disappeared. In addition, everyone's memory about Aichi is lost...Almost all hints have been lost, and Kai starts to search Aichi with "Blaster Blade" and the deck of Royal Paladin, the only existing hint and the proof of their bonds. 
Kai is approached by fighters who call themselves "Quatre Knights". The four---Gaillard, Neve, Ratie and Sarah---are fighters with world-class power in vanguard fights. They control mysterious power and urge Kai to give up searching Aichi. What is their purpose...!? And what is their relationship with Aichi...!? 
Kai, with the new power "Legion" in his hand, fights to take Aichi back. Now, the war has begun! 
 
(Source: Cardfight!! Vanguard Wikia)

Gai Usui