Have you ever wondered how and why certain demons in KNY appear to have more than one BDA?
Well! I'll try to answer that question! Using headcanons, of course, since officially, we haven't received an answer (or if we have, I haven't seen it).
Examples will come to mind, so I might end up forgetting one or two.
Demons that appear to possess more than one BDA have actually undergone an extremely rare mutation upon receiving Muzan's blood.
Demons with the BDA mutation - Type; Transformation
This mutation of BDA allows these demons to assume a new form, seeking to further expand their demonic abilities.
Demons with the BDA mutation - Type; Cloning
These demons normally possess a primary BDA, but they ended up developing this BDA mutation, where they are able to clone themselves.
Demons with the BDA mutation - Type; Merge
The only one I could think of off the top of my head was him, but it's possible to see that after receiving a large amount of Muzan's blood, he ended up developing this mutation, where he can fuse his body with the train
Demons with the BDA mutation - Type; Decrease and increase in size
They possess the incredible ability to increase and decrease in size whenever they wish.
However, for Nezuko specifically, this would be more her main BDA, since the flames she produces appear later in the story.
Demons with the BDA mutation - Type; Summoning
I could only think of him specifically, but Hairo has the ability to summon shadow wolves to assist him in his battles.
How and why can a demon have these mutations? -
Mutations of BDA are extremely rare and difficult to obtain, and depend heavily on the number of humans they ate and the amount of Muzan's blood they received.
A demon can have one to, at most, three BDA mutations.
We have the main/primary BDAs, that is, those they developed after feeding on humans, and the secondary ones, which may come from Muzan's blood being injected into the demon (Enmu) or that were naturally developed (Nezuko).
→ These mutations can also be called "secondary BDAs"