Was told to take my CG Rants/ramble's to the forum, and was referred to this thread.
Spent so much time skimming the thread that I forgot what I was going to start with, so instead, I'll just touch on stuff that was bothering me while skimming:
According the the CG tech books, the Lancelot Albion was absolutely a peer machine to the Guren Seiten, Kallen's assumption that her machine had better specs was wrong. (I assume she just counted the feathers or something, or the Albion needs to turn it's safeties off to match it or something. Lloyd and Cecile developed the Seiten and had at least a month to make sure the Albion could fight it, they didn't just shove their thumbs up their ass the whole time.)
Suzaku was absolutely trying to win their fight, and losing wasn't part of the plan, his internal monologue makes it clear he was baffled that he wasn't winning, and the Re; Manga adds more to his internal monologue that makes it even more clear he was trying to win and couldn't, and admits his loss more clearly too, though he says his loss probably doesn't matter. (Because for some reason Gino just floats there holding Kallen and neither of them do anything after this scene, for some reason, when they're fully capable of landing and going after Lelouch still.)
Presumably Lelouch wanted his 'death' to be more public, with maybe a bit of Lelouch 'betraying' him to make Lelouch seem more despicable for betraying and murdering his own loyal knight, and to focus some of the Suzaku specific hate onto himself, but we don't know.
According to the Novel's (and Suzaku of the Counterattack, though that changes stuff a bit) the Kururugi family was charged with guarding Kaminejima in ancient times by the Geass civ, and Suzaku is so batshit because he has magitech genetic enhancements, which are why he can sense Code bearers and stuff. (like react in terms of microseconds, beat grown men as a ten year old, dodge bullets, shatter steel with his bare hands, lift forklift loads, jump over KMF's, so on.)
Counterattack adds on that people like him are called Regulators and can steal Code's from Code bearer's that are abusing their powers, though that might be Counterattack specific.
Given Suzaku was in a peer machine, with genetic augments and psychic performance enhancing magical steroids and still could only eek out a disadvantaged draw at best (It's notable Suzaku started undamaged and relatively topped off on ammo but Kallen only had three shots left because she wasted most of her RWS ammo during her stupid cat fight with C.C. and Lloyd didn't give the Seiten any back up weapons) it's decently clear Kallen ultimately wins out... in KMF piloting. (What post series stuff we have has Kallen still being considered the Ace of the Black Knights even after they have Suzaku and she actually quit, so)
At everything else, Suzaku wins out for... Numerous, obvious reasons. (Except being Zero, apparently, Zero Kallen in counterattack defeated Britannia and retook japan in like three days off screen.)
Also, seeing some stuff of Kallen's character...
Being honest, I consider her more confusing than Lelouch.
A lot a people don't want to blame Lelouch for his deliberate sabotage of their relationship (because he did, he deliberately and obviously did, and then left it sabotaged because he didn't want her to get close to him) but their fuckups go two ways.
Kallen's bizarre refusal to accept that Lelouch is Zero, that Zero is just a codename for him, hurt her as a character really badly. They turned what should have taken ten seconds into an entire seasons character arc and it just makes her seem incredibly stupid. (I greatly prefer the pre rewrite version of the Kaminejima confrontation scene, personally.)
During the Kaminejima confrontation... Given he went on to admit to causing a horrible massacre with his mind powers, said he was just using the BK, and insinuated he was doing it all to take over the world (planting the seeds for her to believe his Demon Emperor act later) and blew her off to try and call a truce with Suzaku while talking about how him and Suzaku can do anything together shortly after Kallen nearly died trying to protect Lelouch FROM Suzaku... And then threatened to kill "the both of you" with a sakuradite bomb, then looked at Kallen like she was dog shit when she still tried to reach out to him after Suzaku disarmed him and held him down in spite of all of that, I can almost give her a pass for just leaving, given she'd have only lost anyway and regrouping and trying to save Lelouch later was the better strategy.
The reveal in the Granblue Fantasy cross goes vastly better. (she still does the "Zero, not Lelouch" thing, but it's "from now on, you are Zero to me, not Lelouch" and seems more an attempt to just forget about how much an ass Lelouch was at school than anything, she's nothing put polite, differential and subservient from then on, with almost none of her R2 'Tsundere' bits, and keeps going "Lelouch- no, Zero." for a few days before seemingly getting over it. Also notably, Kallen does not give a FUUUUUUUUUUUUCK about Lelouch being a Prince, her classmate, or that he's kind of an ass, it was the "Lol I caused a massacre and abandoned my own army to die, who cares" "I'm picking this enemy soldier who's trying to capture or kill me over my own soldier who is right here" "I'm going to tell one of my soldiers I'm using them for my own gain right to their face without explaining why" "Let's threaten to kill you if Suzaku doesn't do what I want" shit that ultimately had her run off- And it was a near enough thing that the original script had her NOT do it just because Lelouch said her name after he shot Suzaku.)
I've also seen people criticize her for asking him if she could trust him on the Ikaruga and... That's not what she was asking? Nunnally asked Kallen what she was to Lelouch, and Kallen didn't know, so when the chips were down and they were about to maybe die, Kallen asked what she was to him.
It wasn't about trust, it was something she'd wanted to know for a long time and now it was her last chance to ask. Lelouch just gave a very shortsighted and very custom designed answer.
Because Schneizel was planning on killing all of the Black Knights already, so Kallen would absolutely not have been safe if Lelouch had died, even assuming Charles failed to bring about Ragnarok, Kallen would have died in a surprise Fleija bombing shortly after the Damocles was functional.
The fact she left him so easily when he was using a tactic she herself used on her mother is... Frustrating. (And confusing, she'll kill for him, die for him, betray Japan and her comrades for him... But won't tolerate rudeness???)
Her not understanding "Live On" is especially bizarre, because it's so obvious. She doesn't realize it's meaning until she see's Zerozaku, and mentions it in her Character Poem.
Like if you want go full No Sympathy, No Excuses, 100% Blame, Kallen is just a crybaby, a coward and a traitor. She flat out said she 'served Zero, not you" when Zero is just a code name, so she basically said she wasn't loyal to him at all. She made up an imaginary person who never existed and swore her undying loyalty to them, and threw a tantrum and ran away crying whenever the real person wasn't like her pretend one. Then, after admitting her disloyalty to his face, she has the gall to get angry when he responded by cutting her out and not telling her anything. In this light, I legit get scared that they let her mother live with her, what if Kallen hurts her because she back talks about dishes or something? Kallen betrayed and tried to murder Lelouch for less.
But, she's obviously not meant to be read that way, and Lelouch doesn't read her that way, he wouldn't have listed her with 100 Loyalty on his stat board if he did, the writers wouldn't talk about her the way they do, her character songs wouldn't be what they are. It's obvious from the directors commentary and such that it's meant to be a testament to Lelouch's manipulation skills and such, but R2 was the season where the writing for that stuff got really lazy. (See all the comments about Schneizel's strategic ability and intelligence, then what he actually does.) She wouldn't have said all that and then gently folded the jacket he gave her while throwing her own clothes around at random.
At the end of the day, Code Geass Lelouch of the Rebellion was almost entirely made to have Lelouch kill himself (the writers have outright admitted Code Geass was partly born out of one of them basically going "what if we made a show where the protag killed himself at the end?"), and a man who has something to live for doesn't contemplate dying, so Lelouch had to lose everything.
Also, it would be extremely hypocritical of me to spend the whole series telling Lelouch to Just Eat Gilligan with Suzaku and kill his ass because friendship isn't worth all the people that are dying because he won't get out the way, and then turn around and give Kallen shit for being willing to force herself to kill someone she loves because they're declaring a war on the whole world that will kill millions of people. (I'm kinda surprised she didn't assume he got Geassed by his father again, since she knew that was a thing, but Lelouch insinuated he wanted to take over the world all the way back on Kaminejima.)
(The only statement we are ever given in regards to how many people Demon Emperor Lelouch killed is something like "during the two month reign of the Demon Emperor, the world population dropped by 80,000,000", and that this is inarguable, which, given the world population rises, on average, a bit over 23 million in two months, means he killed, globally, 103~ million or so people, not counting how many died from his volcano bomb or Nunnally destroying Pendragon. Knowing that, not only do I consider the ZR an utter, total waste of life that wasn't even remotely worth what it accomplished (since when you cut out all the stupid bullshit third grade philosophy, all it did was get Britannia into the UFN without Schneizel killing them all and making sure not too many people would complain about it (because he murdered them), so, essentially, it made it so Kaguya wouldn't have to yell as much at meetings and made the Britannian Rep play nice), I also can't get mad at Kallen for trying to stop it, because it absolutely was evil, and Lelouch and Suzaku are evil for doing it. Suzaku even admits this in Re; and doesn't disagree with Kallen when she says what he's doing is Despicable.)
I can, however, give her shit for blowing that off and letting Lelouch get away so she can have a pointless catfight with C.C. wherein she wastes most of the ammo for her RWS, or just floating in place doing nothing after destroying the Lancelot Albion instead of getting out of the Seiten and going after Lelouch on foot.
Ultimately, Kallen is a character I like that I have gripes with because of bad writing decisions and because she's... They wrote her to be this super loyal deredere dog girl, and then gave her Chronic Backstabbing Disorder and then pretended they didn't and don't let anyone in universe say anything about it.
I guess, ultimately, she stayed fighting for Japan the whole way, but Japan wasn't what she kept swearing loyalty to, so...
I'm tired of writing this, I'll add more later when I have something else.
EDIT:
A basic idea taken from Kamil The Awesome's up for adoption WIP's: Truth and Reconciliation (Code Geass): Worried for Zero after the liberation of Tōdō, Kallen listens to his audio log from that fateful mission. In doing so, she realizes who Zero is behind the mask and sets into motion a different path, a different life, a different providence for the man within. Canon Divergence after Stage 17.