To be honest, I don't remember Double & Decade Daiki much, other than a lot of standing around and being as confused as everyone else by Wataru's gibbering nonsense, and then joining in for the kick-ass fight at the end. But All Riders is where the character finally sort of hit his apex for me, despite a shaky start (he gets to be the top 3rd Heisei Rider in the tournament because he showed up late?)
That movie has Diend facing down some serious bad guys and joining with the rest of the Rider family at last; probably the two defining moments are his classic Shocker soldier reveal, and then escaping with Natsumi while having to fight off pretty much everybody after the Rider pow-wow goes bad (what'd he expect, hooking up with two nutbars though?) At last, Diend really felt like a proper hero here, as opposed to simply a thief with a heart of gold.
The other big thing about Diend, apart from his characterization, is the fact that he's kind of indirectly responsible for making Narutaki into such a worthless character, given how he took his one cool ability (summon in old Riders to fight Decade) and put a more marketable spin on it. Thus we go from feeling like the real Hoppers and Kaixa have come into he show, to just a bunch of grunting digital copies. To be fair, Diend does it far more over the course of the series than Narutaki, who could conceivably still do it later if he wanted to (re: Hibiki World) so it's more the writing's fault than it is Daiki's.
And yet... I honestly can't say I dislike him. Yes, his character has qualities I don't (and still don't) understand. Yes, he looks like the super heroic equivalent of a PS2. Yes, we still know so little about how he travels between worlds that the "he journeys through the plot holes" theory actually makes complete sense (that said, Episode Yellow does offer up a pretty good explanation of how he gets around Den-O's world, anyway.)
