The main Ryusoulgers are based on (including the joker) the highest suits of cards.
Koh is the ace.
Tyramigo's learning Japanese while still mispronouncing "Melto" makes more sense if you've noticed that all the Ryusoulgers' names (including Canalo and Seto) comprise of syllables that are often heard as part of Japanese names, except for Melto.
The Mummy Minusaur is grown from one Ryusoul woman's desire to finally die along with her mortal lover. Real mummies are infamous cases of people who mostly take all of it with them when they go, as tombs tend to be full of not just all the treasure they hoarded in life.
With a Ryusoul inserted into a Ryusoul Changer and its visor flipped to cover the eyes, the sword of the Ryusoul becomes a "helmet feather" or sorts for the Ryusoul Changer, the design aesthetic being much like how it was a common trend to depict knights or even dragons in knight armor to have a feather atop of their helmets.
In episode 25, it's rather convenient that nobody re-uploaded Kreon's minusaur-infested video back on the internet after Kreon himself deleted it. However, this makes sense. Anyone who watched the video has their souls stolen and was left catatonic. They wouldn't be able to reupload the video if they can't do anything at all.