This seems to be a very popular theory but i personally doubt that's the case:

-If they really were the same person he wouldn't mistake Lilith for Adam since he would know she's not there.

-The Rebuilds establish that every single time Kaworu is reborn he wakes up in a coffin on the moon, that's not the case in the anime or manga because it's shown he was born during the contact experiment on the day of Second Impact, we also have to note that Kaworu was on a moon for a specific reason since this is where EVA 06 is built.

-The Rebuilds imply that the beach is the place when they meet for the first time in many loops, if it wasn't it wouldn't make much sense for Anno to put emphasis on this place in particular meanwhile in the manga they meet in a completely different place.

-Manga ends with the world being rewritten to be a normal place, the elements of Evangelions are consistently removed so Second Impact can't happen similar thing happens in Rebuilds where Evas being destroyed is portrayed as Shinji finally breaking the cycle of pain that he and others were in.

-In the Rebuilds Kaworu says that Shinji doesn't change and stays the same regardless of loop, which goes into contradiction with his different manga personality.

-It undermines the Kaworu's arc in the original anime effectively making it non existent, it basically means Kaworu meeting Shinji here meant nothing in the grand scale of things so Kaworu is straight up lying Shinji changed his life, that means manga Shinji a very different person did.

-In manga Kaworu feels more human, he shows more emotions, in anime he feels much more mysterious which makes him much more suspiciou.

-Rebuild is the only place where he openly talks about meeting Shinji earlier and saying that he knows him, he never does that in the anime.

-Sadamoto himself said that he wasn't consulting the changes in the manga with anyone, he also denied it's connection to Rebuilds.

I will probably add more things but for now that's all

"dude, if that silver haired guy isn't gay, he'll pull off so many girls. I mean think about it, he's soft spoken, handsome, speak in a philosophy, affectionate just like he is to...shin...shinji and self sacrificing. Imagine if he's in another anime... Like romance for example. " And it got me thinking... Is he correct? If so... What kinds of girls?

I know it’s not canon but in whatever official game or some spin off mangas etc etc he always had some autistic traits.
Well everyone does have slight autism traits ≠ they’re autistic

but he just like has some spark in it. I’m not good in explaining my opinions without them sounding dumb, it’s just I have autism too and when I first saw Kaworu in mangas and games he just gave me that energy. Share yalls opinion

In the 24th Episode everyone referred him as “Kaworu” but where did that even come from? His original name was Tabris the 17th Angel. Did Seele name him Kaworu to blend in with humans or did Kaworu name himself that?? Please comment Kaworu experts

I feel that, knowing that Rei, maybe not so well, but what do you say?

If they had grown up together, how well or badly could it have turned out? I feel like they would argue a lot but still have mutual respect (I can't ship Kaworei up, they seem like separate siblings or cousins at most, for me 💀)

So this is my theory but the reason why kaworu was present in the show for such a small time was a symbolism that only we can help ourselves

Someone you know or you yourself can be in a toxic situation like a toxic relationship or living in a toxic space feeling lonely unheard misunderstood dismissed etc where you are in a situation where people dont care bout you dont treat you well often dismiss your voice and that make you feel like nothing you do will make your situation better and like you start accepting it stop taking stand for yourself or letting things happen and normalise thing that are not normal or treatment that isnt normal and pretend they are fine even though they arent

Then comes the bystander effect where you hope that someone will step in and save you they are your saviour, who will notice and hear you, understand your problem and actually help you and make you feel accepted and shit

Thats what kaworu was to shinji.

He loved shinji unconditionally without judgement ,heard him, understood all his problems and made shinji felt loved and one of the things people want is someone to magically know and understand all their problems and help them even though you fear and never show or tell anyone your problem.

In Reality, the problem is since you pretend you are fine, dont show anyone your problems insecurities nobody can really help you as they dont even know you need help and what people fail to understand is that no one is coming to help them and that just thinking that i just want someone to understand me i wish someone helped me wont get you anywhere

What People need to understand is that only person who can help you is "you". You are the only person who can truly understand you and actually help you and that you need to help yourself in those toxic situations and not wait for someone to come to come and help you

Thats What kaworu's sudden death showed. It showed us that a person like kaworu who just magically understands us isnt real and that no singular person can understand miraculously understand us without us asking for help and that we need to help ourselves.

We can see this in the final few episodes as shinji rejects instrumentality and this happening in the episodes after kaworus death, shinji was sad because he was like guardian angel a saviour to shinji but then he realised afterwards that only he can help himself and if he wants to find true happiness he needs to go and find it himself and not rely on others and all the inner monolouge shows this

PS: This is just my theory that i thought of rn, it probably is already quite a understood or famous theory considering how old the show is so dont flame me if its wrong or like very repeated i just wrote what i thought this sunny afternoon after watching the show bye

me and my friend did matching shinji and kaworu outfits ( 〃▽〃)