Addapted from Myanimelist.
Haruhi Suzumiya, a first year student at North High School (in novels 1-8), and second year (in the 9th and latest novel), can be characterized as funny, eccentric, and somewhat unpredictable. In the series, she is introduced declaring her interests in aliens, espers and time travelers and does not care for 'ordinary' human beings. The last thing on her mind is a normal high school life. With a burning desire to find such phenomena in the world, she is generally unapproachable and finds ordinary people boring and uninteresting. As a result of this, many of her classmates choose to avoid and confront her. She deals with her emotions by creating a parallel universe, where she inadvertently releases transparent creatures who tear up and destroy everything that comes into contact with them. Espers are the ones who can stop these creatures by cutting and destroying them, because if these creatures are left alone destroying everything as well as the outer barrier it could affect the outside world. She also likes to dress up Mikuru in random outfits.
Kyon is the first character to engage with her in a serious conversation. From this encounter, she gradually begins to soften up to others and starts to open up with her classmates. Later on, Haruhi took Kyon against his will and forced him to help her start the SOS Brigade, thus she has taken an unknown interests to Kyon later in the series. It is hinted that one of the reasons as to why Haruhi opened up to Kyon was that he resembled John Smith; John Smith and Kyon are the same person.
She is noted as smart, athletic, and naturally talented in any extra-curricular activities. She once joined every activity club in her school and quit because she found them boring, despite pleas by the club members for her membership. Her musical talent was highlighted when she stood in as the emergency lead singer and guitarist, along with Yuki, for the band ENOZ during the school festival.
At first, Haruhi seems very indifferent and cold but underneath that front she is very spontaneous, upbeat, and hyperactive. Her determination, persistence, and resourcefulness drive her endeavors of finding strange phenomenon and promoting the SOS Brigade. Because of her dedication, she becomes hot-headed when things don't go her way. Hand-in-hand with her cleverness and creativity, and a certain ruthlessness, even seeming amorality, she can often get what she wants, an example of such is when she blackmailed the Computer Society President into giving her a brand new computer for the SOS Brigade. Haruhi tends to pursue her goals regardless of who gets in the way, or is hurt, such as Mikuru in the aforementioned blackmail. She also has a tendency to be jealous and angry when Kyon gets too friendly with any girl. Evidently, the universe was almost destroyed and recreated when Haruhi's jealousy was pushed too far.
Haruhi also has a very competitive personality and accepts without hesitation any challenge that crosses her, regardless of difficulty or time constraint. Her constant optimism and over-confidence make her seem arrogant. She has very high standards and won't accept anything less than perfection from herself and her Brigade. She places a lot of trust in her Brigade members, especially Kyon, to get the task done whatever it may be. This can sometimes make her seem selfish and unkind.
She is not shy and often attracts a lot of attention to the point of not minding changing clothes in front of various people, more so when her SOS Brigade is in mind. When it comes to Mikuru she is very physically playful. She often comments on Mikuru's attractiveness and seems to find a kind of enjoyment in exploiting it to her own ends, often through forcing Mikuru into and out of various costumes such as a bunny girl costume as if she were a human dress-up doll.
SOS Brigade role
Haruhi is the founder and "Brigade Chief" of the SOS Brigade, or Saving The World by Overloading it with Fun Haruhi Suzumiya Brigade. She was unable to find any existing club suitable to her preferences. She made the group in order to find and have fun with aliens, espers, and time travelers. Upon creation, she names herself as its leader. Her method of finding and recruiting members is to "voluntarily arrest" them then force them to join.
The SOS Brigade is very precious to her and has stated to Kyon that she has no intention of yielding her position as Chief to anyone else. She will go to any lengths, even by exploiting the members, to bring attention and support for her Brigade such as procuring room and materials, maintaining their website, participating in sports tournaments, organizing activities such as city-wide searches for strange phenomenon among other things. She wrote, produced, and directed the Brigade's amateur-movie The Adventures of Mikuru Asahina for the school festival.
PVMAGKVIDEOS has been using Haruhi as some kind of bait to keep the show a bit followed on the chronoligical events from the past and the future. However he admits the plot has gone off edge for volume 1. However, Haruhi has become more cliche in PVMAGKVIDEOS' AGK Series it's still hard to believe why she's around the show.
From the amature movie that doesn't take place in any form of Leopold's world from the AGK Series. There's somewhat of a connection between the events of Haruhi and the world of Angry German Kid from PVMAGKVIDEOS. Approximately about ten years after the events of Haruhi was discovered chronoligical of the show.
Haruhi has a very different character than what she used to be. She now has no clue what happened to her ten years ago. Nor does she know how she got involved in the world of New Germany. It's all new for her, Since there's no evidence what brought her in the world.
Haruhi used to be acting like she never adored the world and community and ruined it all for them. Jaiden has somehow convinced her too much to stay with the team but most for Jaiden. There's no clue what Jaiden has done that brought Haruhi into the scene of the AGK Production house.
The amature movie, Two Hours (2H) Would've been explained what brought Haruhi into the team. However, the amature movie got delayed and has only made a positive version presumable from the ending
The internet fandom of PVMAGKVIDEOS gave Haruhi the worst character in any AGK series. Due to her lack of trust and only having a perverted mind on Jaiden and anything that she thinks she doesn't give any care for. During the reboot of season 6 it's been noticed Haruhi has been seen two times, one that looks idebtical to Isabella from season 7, and another Haruhi that has the most closest personality of the real one in the anime.
During the reboot season 6 Episode 67, Leopold and Patrick Starfist travel in a strange world that looks it belongs into an anime, Leopold gets hallucinations from his old shows (Likely the Leopold that owns that show haunts the other Leopold from the reboot) Leopold then says they need to go to Haruhi's school campus immediately, as they arrive, Leopold encounters Nagato Yuki who then says someone of your kind has tampered with their world. Leopold and the rest goes inside the school until they see Haruhi chatting with Leonard who escaped by himself from the ship and went into a portal to arrive into ( Season 8 Beta1) Likely a early idea based for season 8 which could've been Leopold disappearing from Germany and instead lives in the Haruhi universe.
There's a high possibility that Haruhi either got flirted to go to the AGK world by Leonard or Leopold's insults. Or Jaiden has once transported himself to the Haruhi universe and kidnapped/flirted.
There are possible theories about her existance into show adapted from what the AGK Series have told through out the Misappearance of Leopold Slikk (Which could be a false rumor according that Dark Lord Leopold Slikk kills off fake and dead series):
-Haruhi has been artificially made from a dakimakura (Body Pillow)
-Jaiden had a transportation accident from the events of Haruhi
-Haruhi has left his own Brigade after the show and has done stuff to the world from ten years after until she came to New Germany.
-Haruhi was triggered to come to New Germany by a mysterious mythical character.
-She might be a hallucination and people who do see her suffer a serious issue.