Just hopping in here to clarify something, but the sad thing about Advent Children Complete is that it IS the complete movie. It's not a "fix-up"... it was what they were originally trying to aim for when they decided to make a full movie instead of a twenty-minute clip.
The first release was unfortunately a problem of time and budgetary concerns caused by the higher-ups at Square, from what we can tell. They pushed the AC team to stick to that one deadline instead of giving them the time to really complete it. Actually finishing Advent Children after the first release was something Nomura and his team were forced to do on the side, which is another part of the reason why it took so long, as other projects and their deadlines took precedence.
So, arguing the always-dicey ground of authorial intent, ACC is really called Advent Children Complete because it's the Real McCoy. It IS the actual fanfic, not a fix-it fic.
As for Denzel... I can see why a lot of people, especially without having seen ACC, would not view him as a necessary character.
However... 1) Sorry, he's not an OC. By dint of the fact that Square made him, he's CC in the Advent Children storyline. So the rules don't exactly apply.
2) Denzel exists as a mental prod in Cloud's poor brainbits for a lot of things, including his survivor's guilt and, most importantly, as a living reminder of what Zack was to him.
Cloud shows concern for Denzel when he finds him collapsed outside of the church, but when you see the clip of what happens right after that, with Cloud putting him up onto Fenrir, he says that Tifa says that he should bring Denzel home. You can see that his concern is just that, and not any deeper. This is where the subtle tipping point occurs.
Denzel, practically unconscious, falls forward against Cloud, and his shaking hand attempts to reach around Cloud and can't. And Cloud, eyes widening, realizes what exactly is going on, and when you see his hand cover Denzel's you can see him going, "This is what it was like for me, too. This is exactly how it was back then." And at that point that Denzel has become important to Cloud, and Cloud, who was perfectly fine waiting for Geostigma to kill him off, suddenly has much more reason to try and find a cure.
You have that whole point where Denzel gets kidnapped/possessed for a bit, but after he comes back to himself, everything after that shows how he's looking up to Cloud as a role model and attempting to protect other people, even going out of his way to do so. And the whole ending where Cloud walks Denzel up to Zack's grave is completely a, "Here's the story, and I'm passing it on to you now" moment.
I think Marlene doesn't quite work as The Kid Who Should Get Sick because, although she's important to Cloud and he wants to protect her, she has much more of a support group, such as it is. She has a family. She has her daddy, and Elmyra, and the rest of Avalanche, all who dote on her. And, as is obvious, she knows exactly who she is and what she wants, and is pretty certain of herself for a six-year old.
Geostigma being what it is, it wouldn't have worked the same on her. There wouldn't have been her turning evil; I honestly think she's too strong of a character, in a sense, for it to have worked. The remnants took the orphans and other children precisely because they were vulnerable children without family to protect them, making them easy to exploit. Marlene got brought along because she was, frankly, in the wrong place at the wrong time, and Loz is kinda too simple to know what to do with her.
Anyway, the writers needed a victim. Someone who has lost his family, is trying to build a new one, and is stuck, confused, and uncertain. I think trying to change Marlene to fit that role would have been worse than creating a new one. So, Denzel fills that role, too.
So, yes, he 1) works to illuminate more of Cloud's character and enables Cloud to grow somewhat, as well as providing some impetus/motivation for some of his actions, whether they're right or wrong; 2) fits in the role of the future protege, which is totally how Square Enix is setting him up; 3) provides an adequate victim for Geostigma.
That said, yes the writing could have been better.
tl;dr--I can't believe I'm defending Denzel but here it is
The first release was unfortunately a problem of time and budgetary concerns caused by the higher-ups at Square, from what we can tell. They pushed the AC team to stick to that one deadline instead of giving them the time to really complete it. Actually finishing Advent Children after the first release was something Nomura and his team were forced to do on the side, which is another part of the reason why it took so long, as other projects and their deadlines took precedence.
So, arguing the always-dicey ground of authorial intent, ACC is really called Advent Children Complete because it's the Real McCoy. It IS the actual fanfic, not a fix-it fic.
As for Denzel... I can see why a lot of people, especially without having seen ACC, would not view him as a necessary character.
However... 1) Sorry, he's not an OC. By dint of the fact that Square made him, he's CC in the Advent Children storyline. So the rules don't exactly apply.
2) Denzel exists as a mental prod in Cloud's poor brainbits for a lot of things, including his survivor's guilt and, most importantly, as a living reminder of what Zack was to him.
Cloud shows concern for Denzel when he finds him collapsed outside of the church, but when you see the clip of what happens right after that, with Cloud putting him up onto Fenrir, he says that Tifa says that he should bring Denzel home. You can see that his concern is just that, and not any deeper. This is where the subtle tipping point occurs.
Denzel, practically unconscious, falls forward against Cloud, and his shaking hand attempts to reach around Cloud and can't. And Cloud, eyes widening, realizes what exactly is going on, and when you see his hand cover Denzel's you can see him going, "This is what it was like for me, too. This is exactly how it was back then." And at that point that Denzel has become important to Cloud, and Cloud, who was perfectly fine waiting for Geostigma to kill him off, suddenly has much more reason to try and find a cure.
You have that whole point where Denzel gets kidnapped/possessed for a bit, but after he comes back to himself, everything after that shows how he's looking up to Cloud as a role model and attempting to protect other people, even going out of his way to do so. And the whole ending where Cloud walks Denzel up to Zack's grave is completely a, "Here's the story, and I'm passing it on to you now" moment.
I think Marlene doesn't quite work as The Kid Who Should Get Sick because, although she's important to Cloud and he wants to protect her, she has much more of a support group, such as it is. She has a family. She has her daddy, and Elmyra, and the rest of Avalanche, all who dote on her. And, as is obvious, she knows exactly who she is and what she wants, and is pretty certain of herself for a six-year old.
Geostigma being what it is, it wouldn't have worked the same on her. There wouldn't have been her turning evil; I honestly think she's too strong of a character, in a sense, for it to have worked. The remnants took the orphans and other children precisely because they were vulnerable children without family to protect them, making them easy to exploit. Marlene got brought along because she was, frankly, in the wrong place at the wrong time, and Loz is kinda too simple to know what to do with her.
Anyway, the writers needed a victim. Someone who has lost his family, is trying to build a new one, and is stuck, confused, and uncertain. I think trying to change Marlene to fit that role would have been worse than creating a new one. So, Denzel fills that role, too.
So, yes, he 1) works to illuminate more of Cloud's character and enables Cloud to grow somewhat, as well as providing some impetus/motivation for some of his actions, whether they're right or wrong; 2) fits in the role of the future protege, which is totally how Square Enix is setting him up; 3) provides an adequate victim for Geostigma.
That said, yes the writing could have been better.