Top Definition
\ˈhir-(ˌ)ō ˈsin-ˌdrōm\ (noun)

1. An infrequent, but nonetheless lethal, psychological occurrence in a male character from any of a variety of TV media, particularly those found in Japanese anime, in which the character in question experiences a drastic and negative change in personality due to the on-comings of a female character, or in order to gain a female character's interest or attention. This sudden change in behavior can lead to a falling-out with any number of friends, peers, or companions; resignation from or disinterest in a formerly favored career track; moodiness or downright cruelty; and, in general, the taking of sudden and drastic actions that may work as a plot device but are otherwise completely out of line. While these alterations in a character's attitude are usually temporary, they can, on instance, ruin a character forever. If the character in question is a "fan favorite," then his sudden change for the worse is all the more upsetting to any dedicated fan. The term originates with the character Nakano "Hiro" Hiroshi of the anime Gravitation, and how in his romantic pursuit of the character Usami Ayaka, he turns into what many fans consider a complete and utter douche, and takes his unrequited feelings out on his bandmates and friends.

2. When a male anime character previously assumed as homosexual abandons all prospects of a relationship with a male because of the influence of a nearby female, enormously disappointing the avid BL fan.
"Wow. First Spike was a total asshole towards Jet, then he abandoned his nakama without even thinking."
"It's because Spike wants to find Julia so badly. He must have serious Hiro Syndrome."
by ack-gabbit November 20, 2009

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When a nearly-flawless male character in any form of media (usually a well-liked character or even the fanbase favorite) has a sudden change in personality as a result of a relationship with a female character. This change in personality is "a turn for the worse," making the character unwelcome in the media, and often unliked by the majority of fans.
"I don't get it! Hiro's acting like such a dick just because of that bitch Ayaka!"
"Oh, it's because of Hiro Syndrome."
by TRAW!!! November 20, 2009

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When a totally awesome male character in a book or show starts acting very asshole-ish because of a girl or woman.
"You're just being a jerk because of that stupid Haruhi."
"What, do I look like I have Hiro Syndrome?"
by QUACK*QUACK November 18, 2009

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When a guy in anime, usually a well-loved or attributed character, goes after a girl who doesn't want him, whose rejection makes him act like a total asshole. Usually applies to gay men in Boys' Love anime who act all hetero over one specific girl, much to fans' dismay; however, it can also apply to canonically heterosexual males or males in non-BL anime when fitting the criteria of NICE GUY TURNED RETARDED THANKS TO BITCH. Often used as a plot device in fan fiction.
"Wow, did Steve totally just ditch his best friend Michael for Mikayla or what?"

"He's only acting like a jerk because of Hiro Syndrome."
by THEBEARJEWPWN'DYOUANDLEFT January 26, 2010

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