Demyx has been in Nautilus for a long, long time. For those of you not familiar with Kingdom Hearts, Demyx was a Nobody, aka one of those people without hearts. But he wasn’t any Nobody- he was also a nobody with a lowercase ‘n’ for most of the characters. He was unmotivated, lazy, a coward, and was generally not considered very good at what he did-- not that he did much.
In Nautilus, this has changed quite a bit. He arrived shortly after being killed by Sora, and was mostly just glad to have the chance to continue living. Change was slow for him for a while, but he ended up making a few friends that were a very good influence on him. When Isis came for the first time and offered all the Nobodies hearts, he wanted to try and see if he could get it without her help, and asked his friends for aid instead. It worked, and this is where he really started changing.
Going from having no emotions to having ALL of them is a huge switch, and one Demyx wasn’t entirely prepared for. He was pretty unstable for a while, but he had a saving grace. Even though he pretty much had to start from the beginning in emotional and moral development, his heart had been Bended by people who had cared for him, and who had a good moral code. Because of this, change came a bit easier. He began to learn the basics between right and wrong all over again, even though he knew some of them in his head already.
I swear the backstory here was kind of relevant, because it explains the biggest change I feel Demyx has gone through. He started picking up on the fact that when a friend got hurt, he felt bad. He felt even worse when he could have helped, and just... didn’t. He didn’t go to Kalliste, and feels awful about that even now, because Axel (among other people) died or were really hurt and he wasn’t there to help. He started stepping up to help friends out when they needed it, even learning to how to use more types of magic in order to be able to defend himself and others. The fact that he’s actually working in order to help people? That’s a huge step for him.
When he became an Angelli, he wasn’t as outwardly active as most others for a pretty basic reason- he had no idea how to be. Heck, he didn’t know what to do with the chunk of responsibility he’d been handed for a long time. He wasn’t really equipped to be a leader, and since he’d been constantly told for so long that he wasn’t good at much of anything he didn’t think he could help at all. Still, he learned the new powers he got with being an Angelli and wound up putting them to good use. Every time there’s a reality storm and he’s not affected, he’s always teleporting people who are affected, or who just want to get away, to Helix. His range of help is still pretty small, sure- he mostly sticks to friends, or friends of friends, or people who ask him for some reason. But that’s mostly because he knows there are other Angelli out there helping out too, and he’d feel... weird offering help to someone he didn’t and who already had a friend of their own who could do the same thing. As Ashura, he might freak out a little at first, but he’d adjust to the responsibility and reach out more, since there definitely aren’t as many of those.
Recently, he’s even tried to help with newcomers. If there are people coming in who need Bending lessons, and no one else has offered, he’ll offer up his services too. (This hasn’t happened often, but it’s a start.) It’s hard for him most of the time unless he can find something to strike up a conversation about (like music), but he knows that people need a little help when they first arrive and is willing to do it. During this past metaplot, he’s also been trying to help with finding people and coming up with explanations, even if he’s not very good at it.
During the Dark Aeon event, Demyx served two primary purposes: 1.) make sure Axel didn't do anything really stupid. Which he tried to, on several occasions. Seeing as all the residents of the castle had gone missing, Axel was freaking out and Demyx managed to help calm him down enough not to do things like kill himself in order to find them. 2.) He made a lot of effort to try and figure out what was going on, and once they figured it out he immediately went to help. He considers Nautilus his home, and wanted to do whatever he could to protect it.
There have been some other miscellaneous changes that I don’t know how to put into paragraphs so they’re just going to go here. He’s starting getting more tactful, and while he was pretty much a wuss before, is learning how to stand up for himself, in fights and just in conversations. He’s also learned enough to give semi-decent advice, sometimes. He’s managed to make a best friend in the form of Zack, who has encouraged him to focus a little on fighting, and to focus more on the positive side of thing, which he managed to do even in the face of Zack falling back to Sleep. Also, whatever’s going on with Axel. That’s a change, and it’s one he still doesn’t know what to do with. But, and here’s another change, he’s not running away. For things he didn’t understand or wasn’t sure about before, his instinct was to just get away from it. But he’s learning, and he has learned, and he knows the only way he’ll be able to really help people is to keep learning, even if it’s a hard path. Does he want to run sometimes? Hell yes, but he doesn’t. He might not be the one with the most leadership qualities, or the most intelligent, and he’s sure as hell not perfect, but he’s trying. And he’s changing.