The "Wano" Arc of One Piece has been a story of the Kozuki Clan's revenge on Kaido and Orochi for what they did to Oden and the people of Wano. The Red Scabbards abandoned their lord so they could one day reunite for the chance to take down his killer.
Though the Red Scabbards were able to surprise Kaido and do some damage, the Emperor of the Sea proved too much for them. The rooftop battle ended in defeat for the vassals of Oden, and they were sent down below to recover their strength. Once they deal with a trick by Kanjuro and head out again, they are confronted by Orochi before swiftly cutting off nearly all of his remaining heads. After this, however, Denjiro is neither seen nor heard from again.
The next time One Piece readers see the vassals is in Chapter 1012 when they all split up to help in different locations -- but Denjiro is already gone, having simply vanished between Chapters 1009-1012. One possibility is that he went to help Hiyori. Though the vassals do not know she is on Onigashima, she was the silhouetted woman who treated them after their fight with Kaido. Hiyori could have woken Denjiro first and asked for his help -- or gave him a task to do -- then left to go hide in waiting for Orochi. Denjiro was the one who watched over her after her parents' death, so they share a special connection.
What could he be up to though, and was it his plan or Hiyori's? Hiyori is currently with Orochi, standing over him as he burns from the fire Kanjuro started. She has been by his side for years, pretending to be a geisha for the man who killed her parents. It's possible she knows of something Orochi is hiding on Onigshima and has tasked Denjiro with finding it. It could be some kind of treasure of the Kozuki clan, or maybe something of her father's that was stolen by Orochi.
Another possibility is that Denjiro is looking for Kaido's Road Poneglyph. When the Scabbards first made it to the island, Denjiro ran into Sazaki, one of the Tobi Roppo. He says to Denjiro, "I heard you weren't coming this year!!" implying that he had been in the area multiple times before. Denjiro already had knowledge of details like the Torri Gate, so it's clear he has done some scouting of the island. Since this is the case, he could also have scouted the interior of Onigashima during one of the Fire Festivals and found the room where Kaido hid treasures like his Road Poneglyph.
What use Denjiro would have for this -- other than to give it to the Straw Hats -- is unclear, but he has a knowledge of the castle that no other members of the samurai do. Since Orochi escaped Hiyori's trap in One Piece Chapter 1047, Denjiro could find the room they are in and save the day before finally revealing what he's been up to. Hopefully he has been putting his skills to good use to help the raid succeed.
The next chapter of One Piece will be available on Sunday, May 8th.