Best girl syndrome is when someone feverishly believes that their favourite character in fiction (typically a female in anime, hence the name) is the best character and will be stubborn in saying that theirs is the best and everyone should like it, and that if they don't they simply say that they know them better and say that the other person is in denial, not considering the fact that other people have different opinions.
Guy 1: god I love insert anime waifu here
Guy 2: nah is insert different anime waifu is better.
Guy 1: fair enough, I just like mine better.
Guy 2: you don't seem to get it, they are better.
Guy 1: ...to you maybe, but I still prefer anime waifu1.

Guy 2: you just aren't giving them a chance! Stop being stubborn about it!
Guy 1: and you have Best Girl Syndrome
by Undert23 November 8, 2019
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YAGOO / Motoaki Tanigo / CEO of Hololive
Man 1: Who is your favourite hololive membe?
Man 2:you already know who Best Girl is.
Man 1:True.
by March 10, 2022
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Some people think the Bakadere childhood friend is Best Girl.

Some people think the Kuudere bookworm is Best Girl.

Some people think the Tsundere shorty with daddy issues is Best Girl.
And some people think the self-aware Yandere is Best Girl.

They all have their own reasons, and it leads to some heated flame wars.
by Someone who kinda exists August 8, 2021
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A character aside from the main character, irregardless of gender (don't worry, no homo), particularly in an anime that has the most influence in how the plot moves along the series.
Characters that best personify this include: Speedwagon in Jojo's Bizarre Adventure and Kongming in Paripi Koumei.

Speedwagon is best girl
by Ayame Ao June 27, 2022
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