Alkaid. One half thinks she's The Scrappy, the other half is in love with her.
Shino might count as well, since some think she's a complete bitch, and others love her.
Complete Monster: Sakaki was merely annoying and overzealous at first, but later revelations removed any kind of sympathy fans might have had for him. Specifically, the revelation that he had been manipulating and controlling AIDA behind the scenes, and that his ultimate goal was to use AIDA as mass mind control over all of humanity. But the biggest blow against him was his treatment of Atoli, who had long been one of his strongest supporters. Turns out he had met her on a suicide website, preyed on her fears about being alone and considered worthless, effectively brainwashed her into depending on him, and then Mind Raped her to bring out the power of her Epitaph. When all of his plans fail and he gets infected with AIDA himself, he then spends the first half of the final volume putting together a sadistic tournament for the sole purpose of making Haseo and the rest suffer (harming quite a few others in the process), and laughing at the pain he's causing.
Critical Dissonance: Most Critics gave GU a Low to Medium score, most who have played love the game to the point of giving it high to perfect scores. Mainly due to the "fresh" gameplay compared to the older games, as well as more likeable characters and story.
Disappointing Last Level: After testing Skeith's ultimate form, the rest of the Final Boss fight is done with the regular playable characters. Futhermore, most of the next fights against doppeldangers and the final one will barely pose a challenge if the player already defeated the doppelgangers from Disc 1 and 2.
Ensemble Dark Horse: Alkaid and Azure Kite. Alkaid was originally slated to only have a brief appearance in Volume 1, but fandom demand and the creator's preference changed that.
Fridge Logic: How come no one ever brings up ripping off their headset or unplugging their PC?
In the R1 games being anywhere near one of the phases made players begin to have spastic fits such that they are physically unable to take their hands off the controllers unless there's someone in the room to do it for them, like Mai and Tokuouka in the first volume of Liminality. The same possibly holds true for anyone in the presence of AIDA or the Avatars.
Why do Atoli's parents take her to the doctor, confirm she has Doll Syndrome, and still let her play on the computer? The answer to that could be Fridge Horror. We later learn how isolated Atoli is and it's heavily implied from her pre-avatar battle that her parents are at best neglectful.
If Aina was so sick in the hospital, why did doctors allow her to play an online virtual reality game so much?
For that matter, why did doctors let someone who fell into a coma for several months after playing The World have a computer *again* so they could continue to play The World when she woke up?
Funny Moments: Some of the "Marriage/Promise" endings, even more so if you pick someone like Azure Kite.
In Volume 1, when Pi beats Kuhn for his womanizing before the tournament against Alkaid starts.
Volume 4 adds another Marriage ending with Ovan.
Last Recode adds a new mode called "Parody Mode" who changes the dialogue in some cutscenes like Haseo being rude to women, Yata being desperate to have a girlfriend and the Azure Knights being a death metal band among others.
Game-Breaker: The Doppelganger Items and the pair of guns that Haseo gets in the Forest of Pain.
This also includes Haseo's ability to juggle enemies with his guns, semi-infinitely if the player is quick with the reload button.
The DG-Y's ability. Combined with Super Charge, it essentially eliminates Skill Trigger recharge.
Quick Rengeki or Jet Speed Attack + Awakens also fits in this trope. Awakenings are so broken that people have actually complained about how gamebreaking they are.
Starting a new game in 1 if you have the clear data for 2 on your memory card allows you to start the game at level 35 even after being Data Drained. For the record, the PKs who kill you at the start of the game are level 10, and the crazy bitch Bordeaux is only level 27 when you have to fight her in the arena. It's possible to beat every single mission up to the battle with Alkaid without having to level yourself up at all. But be warned, your allies will be horribly weak.
There might be a reason why you are the only player who can use a Broadsword in your party: even all by yourself you can juggle most small sized enemies and "player" enemies until they are dead or enough to finish with a Skill Trigger.
Equip a Broadsword with rapid attacks. Attack an enemy with the Broadsword and chainsaw for the extra hits. Block immediately afterwards, which resets your attack without the cooldown and allows Haseo to juggle enemies like there's no tomorrow - and since you're using the rapid attacks, you'll build up the combo meter quickly, allowing for almost flawless Rengekis, gaining both combo and Rengeki morale bonuses.
For one step further, do the above with the Divine Awakenings starting in Volume 2. At the right level, every second or third fight is just a Divine Awakening, which does so much damage it's ridiculous; weak Divine Awakenings can still do enough damage to wipe out an entire group in one go.
Damage Change + SP Regen could also be considered a Game-Breaker, although to a lesser extent. It's pretty damn easy to break this game.
Atoli can become one. Damage Change, SP Regen, Skill Boost (for Rig Geam, which stacks with SP Regen), and Dancing Haze combine to create an unstoppable wall. Atoli is the only character that can use Dancing Haze - and it gives a 2-slot armor customization that may very well be one of the best caster customizations in the game, on a weapon. If she somehow takes some form of physical damage, she's able to heal herself almost immediately afterward.
Growing the Beard: Vol. 1 was fine but was rough around the edges and many just couldn't get past Haseo's more abrasive moments. It was also considered to have some slow pacing and few stakes despite the threats in AIDA. Vol. 2 featured more streamlined gameplay mechanics and fixed various problems the previous vol had. The story also greatly picked up as the stakes became greater and Character Development really set in not just for Haseo but almost everyone else.
Heartwarming Moments: Volume 3 as a whole is full of them. The ending cutscene with Haseo running after Atoli is one of the milder examples.
Haseo also has some moments with Ovan that gets even more intimate in the manga.
And dear God, Piros the 3rd. "Your love makes me hot!" Also another case of Do Not Want for Haseo.
Kuhn as well. Send him the Flower Gift card and he'll think Haseo is hitting on him.
Silabus, again with Haseo. He's one of the easiest to upgrade his relationship status, and not just because he's in Haseo's party for half of volume 1; it's possible to get him to max-out less than one-quarter of the way through Volume 2. Some of his e-mails suggest a stronger devotion alongside his obvious admiration, and his Promise Card scene is rather...interesting. It should also be noted that Silabus could be considered a Spear Counterpart of Atoli, the closest the game gets to a canon love interest.
Silabus: Hey! You scared me running off like that! (to Haseo after chasing Ovan in Lumina Cloth before meeting Endrance)
Magnificent Bastard: Ovan was once an ordinary hacker playing The World who had the misfortune of getting possessed by AIDA, who forced him to put his sister Aina and other players into comas, becoming the notorious PKer Tri-Edge. To save Aina and the other Lost Ones, Ovan conspired to trigger the Rebirth by manipulating Haseo into fighting and defeating the seven other epitath users, including Ovan himself. Ovan effortlessly manipulates Haseo, the cruelly ambitious Sakaki, system administrators, the World's AI guardians, and even the AIDA possessing him, without revealing his identiy as Tri-Edge until he desires it. His plan to trigger the Rebirth goes off without a hitch and Ovan saves Aina and the Lost Ones at the cost of falling into a coma himself.
Memetic Mutation: The fandom isn't really large enough to have many cases, but...
Moment of Awesome: Too many examples to list. Haseo's arrival as a PKK in Volume 1, the four-way Avatar summoning sequence against Sakaki's AIDA, Atoli telling off Sakaki after he betrayed her with Mind Rape...
Narm: Summoning Skeith the first time is a pretty cool moment. It crosses into Narm when Haseo says the exact same thing, with the same inflection and tone and everything every subsequent time.
Never Live It Down: Haseo was a huge jerk in Vol. 1, with some exceptionally strong Kick the Dog moments, but he also had plenty on the opposite spectrum that kept him from being completely loathsome and he mellowed out towards the end when multiple characters called him out on it. And that doesn't even include his later Character Development. However, detractors on youtube will often cite only his more cruel actions in Rebirth when talking about his character without talking about his redeeming aspects. Reviewers who have only just restarted the game in their reviews and mention just how unlikeable he is for most of it in spite of his other moments.
The Dopplegangers. When one shows up on the map and notices you nearby, it starts to slowly and confidently walk towards you while an increasingly scary music that alerts you of your impending doom starts to play and grows louder the closer it comes toward you. The screen also starts flashing red, and more so when the enemy closes in on you.
Entering in a battle to escape from it is useless like when you get to fight PKers he'll, literally, walk in with his shady appearance.
Innis in vol. 2... "I want you to look at me... NOBODY EVER LOOKS AT ME!"
Skeith beating the hell out of Magus in Volume 1. Haseo panics.
It happens earlier with Alkaid, too. Haseo is speechless and horrified when Skeith appears on it's own and appears to be trying to scream when Skeith attacks Alkaid.
Sakaki mind-raping Atoli may count too, considering Atoli's reaction makes it seem as if she's being literally raped.
Haseo, Pi, and Atoli defeat an AIDA-infected Sakaki, and Ovan shows up to congratulate them... and shows them the truth about Tri-Edge by violently cutting down Pi and Atoli in one stroke each' while creating the Tri-Edge insignia around Haseo. May count as a moment of Creepy Awesome.
Polished Port: Last Recode gathers together all three games, plus a new fourth one, all in one package that runs at a beautiful 1080p 60FPS, and adds some fixes to Rebirth that Reminisce had. That said, the port does have some quirks, since many effects were made with the original 30FPS in mind, and weren't redone to put the new framerate into account. Still, a nice option for playing the games, be it for a revisit or if you're playing for the first time.
The Scrappy: Piros the 3rd is generally agreed as this. Several others can fall into this trope as well, but for the most part they more often fall under Base-Breaking Character and generally fan reaction to their later character development redeems them. Piros, however, is simply outright despised for being an obnoxious Flat Character.
Squick: Hiiragi probably isn't aware that the Sakaki he's fallen madly in love with is in fact a psychotic ten-year-old boy putting on an act, but their relationship is squicky enough even ignoring that...
AIDA <Oswald> in Reminisce. It has several difficult to dodge and very damaging attacks that can easily rack up about 1/5 of your entire life bar and if you get careless you might get hit many times on a row.
AIDA <Victorian> in Rebirth. Destroying its protection isn't (quite) the challenge, Data Draining it is. Victorian moves erratically all over the place while spilling bullets that will cancel your Data Drain gauge entirely if one so as much scratch you. Failing to Data Drain it in time means the boss will have around 1/3 of his life back in a battle that might have taken about half of yours.
The Azure Knights, a memorable late game throwdown against all three of Aura's knights in Hulle Granz Cathedral. This battle marks a notorious difficulty spike in Last Recode, as while the game in general was made easier, these guys still pose a considerable challenge without preparation or level grinding.
Cubia, as well, at least in the Avatar Battle portion. It's not so bad until near the end, when Cubia whips out it's strongest attack, Chaos Gehenna. This is even considering that Cubia is the Final Boss and technically you're fighting it's first (two?) form(s), and that ONE SINGLE ATTACK can easily send you back to the very start of what is already a long battle.
They Wasted a Perfectly Good Character: Gabi and Zelkova get a lot of build-up in the first half of Volume 1. Gabi in particular gets a whole cutscene with him, Haseo, Kuhn, and Bordeaux (amidst detailing some of Kuhn's background), is implied to be a rather strong player, and gets multiple forum responses and some scenes outside of the arena fight that imply he's going to be a major character. Gabi fades out of importance pretty quickly during Volume 2 despite this build-up, while the hinted Zelkova/Sakaki feud becomes part of Volume 2's main storyline and extends into Volume 3.
Video-Game Movies Suck: .hack\\GU Trilogy tries to cover the game of three games in an hour and a half film. Sufficed to say, outside of being watched as an actionized popcorn flick, it fails in that regard.
Wangst: Haseo gets a lot of this in the first game especially when he throws angry tantrums at Atoli for enjoying the game. It is far more exaggerated in the manga and the CGI film when Haseo is given so many creepy expressions that he looks like a molester.
What an Idiot!: Some of CC Corp's decisions are so bad you'd think they were trying to get The World shut down, but kowtowing to Sakaki an Ax-Crazy ten-year-old boy drunk on AIDA and making him a system administrator is in a category of stupid all on its own.
The Woobie: Several. To name a few, Atoli and Sakubo.