It is hard to talk about Joker without talking about another of Nobuyuki's character from his other series. And that character is Nanashi from Märchen Awakens Romance or more commonly known as MÄR. Nanashi is one of the main characters and a protagonist of the story. A part of Team MÄR, he's the leader of the Luberia Guild of Thieves. And he bears a striking resemblance to Joker.
If you have finished reading the Recca no Honoo manga, you would know that Joker's "death" is really unclear. We really don't have a body to be a proof of it. He gets stabbed in the torso by Kadotsu, opens a blackhole, sucks Kadotsu and himself into it and vanishes. What happened to him after being sucked into the blackhole? Fan speculations say that he was sucked into the world of MÄR and now he's known as Nanashi. If you read MÄR, there really are a few things that could prove this theory.
When Joker was sucked into the black hole, he has long blonde, flowing hair, is wearing light colored baggy pants and has a black wristband and a bracelet on his right arm. And like Joker, Nanashi also has the blonde, flowing hair, baggy pants and the black wristband and bracelet on his right arm. They also have the same body build and both of them have their eyes covered most of the times, though in Nanashi's case, we get to see his eyes more than Joker's. Now when you look at their main weapon, there's one big similarity that could be seen there. One of Nanashi's weapons (or arms as the weapons in MÄR are called) and the first that we see him use is his Electric Eye. The Electric Eye is a pair of rings that Nanashi wears at each hand. It's similar to Joker's Taishaku Kaiten in the sense that both the Electric Eye and the Taishaku Kaiten bears the image of a human eye. Another of Nanashi's arm is his Griffin Lance. And like Nanashi with his lance, there are times that Joker uses the Taishaku Kaiten as a lance giving us the reason why Nanashi has the skill to weild such a weapon.
When Ginta, the main hero of MÄR, and Babbo, the living arm of Ginta, met Nanashi, Babbo, having a keen sense of smell, told Ginta that Nanashi has the same scent as him. This could be taken in a lot of ways but more than likely it tells us that like Ginta, Nanashi is someone that came from another world or dimension, possibly Joker coming from the same dimension as Ginta.
When Ginta arrived in the MÄR dimension, he learned that he's stronger than a normal MÄR citizen since he came from another dimension and MÄR has lighter gravity than from where he came from. When Nanashi first fought in the cave, he was able to stop the attack of Oruko, a henchman of the Chess no Koma who is double the size of him. Even Dorothy, a witch who knows a lot about arms, is surprised by his strength. At first she speculated that Nanashi used a strength-enhancing arm but she negated that by saying that she didn't feel any synchronization with an arm happening. This means that Nanashi has enhanced strength like Ginta which is rare for a normal MÄR citizen and proves that he isn't of the same dimension.
Also, we get to see Nanashi's story when Galian, the former boss of Luberia, talked about the time when he saved him. He came upon Nanashi lying on a road while he was severly wounded and offered to heal him with a holy arm. During that time Nanashi has no recollection of who he was and have no idea what a holy arm is that Galian mentioned. When he was fully healed but still can't remember who he is, Galian gave him the name Nanashi which means no name. When we last see Joker before he goes into the black hole, he was stabbed by Kadotsu straight through his torso. This could be the injury that made him lose a lot of blood and cause him to lose his memory as well. And the fact that Joker is new to the MÄR dimension is the reason why he doesn't have any idea what an arm is.
These are a few points that could prove to us that Joker could be Nanashi. But if ever it's true or not, we wouldn't really know until Nobuyuki-sensei proves it otherwise. All we could do until then is to speculate.