I noticed a very interesting parallel between Delta and Drum while watching GT. when Drum talks to Dragon before battle, it's Dragon who's relying on Drum - Dragon always says "*You* aim for this" or "I know *you* can do it" and stuff like that, but it's the other way around with Delta. Delta is the one relying on Diabolos - asking if *Diabolos* is satisfied and telling *Diabolos* where to aim. their relationships with their beys are very similar but also very different and i think that's really cool
I think it shows how afraid Delta is to rely on others, because he does everything to get stronger alone, and if Diabolos isn't satisfied, then it's not enough. Diabolos specifically told him not to let Drum get in his way when battling Arthur for the first time - and Delta obeyed. Delta doesn't want to let Diabolos down - and while Delta is the blader in control here, it really is Diabolos who's pulling the strings.
Again, it's the opposite with Drum - Drum is always looking towards his friends and relying on them and Dragon, but he feels confident enough to stand on his own and go after what *he* wants. Dragon is more of a guide to Drum. The ability to rely on others is really what Delta lacks and it makes for such interesting parallels between the two.
Delta is really only thinking about what Diabolos wants and yes, he's making Diabolos stronger - but Delta himself isn't getting stronger at all, and the fact that it takes Diabolos breaking for him to realize that is actually really sad. Delta loses the one he relied on - he loved Diabolos more than anything - and Delta depended on Diabolos to feel stronger and gain power, so without Diabolos? Delta feels nothing.
When Dragon broke, Drum was able to think about what he wanted as a blader and that's to make Dragon the strongest in the world - and Aiga recognized that, since he's had Achilles have the same thing happen - so it shows it's not Drum only relying on Dragon, but Drum can stand alone in a much different way than Delta can.
Adding onto this, once Diabolos is revived, Delta says “This is MY Diabolos.” He’s finally taking things for himself - Delta is fighting for himself *and* Diabolos, not just Diabolos. Delta also finally lets Drum hug him - albeit a bit disgruntled. Delta is standing on his own, but now he knows he wants someone to support him - shown when he smiles at Drum’s encouragement. Delta is now speaking as the blader - not the bey. He isn’t saying “Diabolos is going to beat you”. He’s saying, “I’m going to beat you.”
Delta has moved past being Diabolos’ vessel. He’s the one who calls the shots now, and he knows it. Instead of saying things like “Diabolos is going to absorb your light”, Delta is saying exactly what he wants, and he knows he’s going to get it. He has friends behind him - he’s no longer afraid to disappoint Diabolos because he became one with Diabolos. The two are now one and the same. They are working together. And Delta is directly defying Diabolos.
When Diabolos says “We’ll attack harder”, the old Delta would have nodded and did what Diabolos said. This time, though, he says “No, we have to find another way. We’re doing things my way this time.” Diabolos isn’t commanding Delta, and Delta isn’t being guided by him either. They’ve formed a completely new bond and instead of Delta relying heavily on Diabolos, they’re relying on each other. Diabolos still wants power - but Delta acknowledges that and goes “Okay, but we’re doing it *my* way and you’re going to trust me.”
It really does show that Delta still loves Diabolos more than anything. But Delta has both feet on the ground now - they’re still fighting together - Delta just has much more control, and a much stronger will to take things for himself - not just Diabolos.