Emi Sendou is the younger sister of Aichi Sendou, the protagonist of the series, and is a recurring character throughout the Cardfight!! Vanguard anime, including the Asia Circuit Hen arc. She is typically recognized for her caring and responsible nature, often acting more mature than her older brother and worrying about his well-being. A dedicated student, Emi is frequently seen encouraging Aichi to complete his homework and wake up on time, taking on a role similar to a concerned parent. Her concern for Aichi lessens as she witnesses how the game of Vanguard helps him grow from a shy, timid boy into a more confident individual. Despite this, she continues to accompany him to many of his important matches at Card Capital and regional tournaments, largely to offer her support and keep an eye on him.

During the events of the Asia Circuit Hen, Emi remains a steadfast supporter of Aichi and his team, Team Q4. While she stayed behind at Card Capital during the first two stages of the circuit in Singapore and Seoul, she traveled with the team to cheer them on during the later stages in Hong Kong and Japan. Her presence serves as a familiar and grounding influence for her brother amidst the high-stakes competition against powerful international fighters.

Emi is shown to be a quick learner and a naturally talented Vanguard fighter in her own right. After being introduced to the game at Card Capital, she learned to play using a loaned deck from Misaki Tokura. She quickly developed a personal preference for the Bermuda Triangle clan, which features units with a musical, idol-themed aesthetic. She finds the artwork on these cards particularly appealing, often describing them as cute, and builds her own competitive deck around them. In a notable achievement just before the start of the Asia Circuit, Emi won the junior division of a tournament held at Card Capital, demonstrating that her skill is a serious force to be reckoned with and not merely a hobby.

Key personal relationships define much of Emi's role in the story. Her primary bond is with her brother, Aichi. She is one of the first to notice the positive transformation Vanguard brings to his life and is fiercely protective of him. She also shares a close friendship with her classmate Mai, with whom she is often seen. Another significant relationship is her one-sided dynamic with Kamui Katsuragi, a fellow fighter who harbors a crush on her. For most of the series, including the Asia Circuit Hen, Emi remains completely oblivious to Kamui's feelings and instead encourages his interest in her friend Nagisa. Over the course of the series, Emi becomes more independent, no longer just an observer of Aichi's adventures but an active participant in the Vanguard community. By the time of the later Link Joker arc, she takes the initiative to establish a Vanguard club at her middle school, Miyaji Academy, solidifying her own identity as a dedicated fighter and leader among her peers.