Large stature, red skin
Demons are a currently unseen species in BlazBlue.
Although unseen, demons have been fought against by the likes of Hakumen at some point before the events of BlazBlue: Calamity Trigger.  The only details about their appearance is that they are large and bright red.
Iron Tager was injected with demon cells by Kokonoe during an operation which saved his life. They massively changed his appearance, causing him to grow to an incredible height and for his skin to permanently turn the same bright red which the species exhibits. The cells can stimulate the functions of living creatures, but the effect is immensely strong and will kill most people who are injected with it. 
Demons seemingly inhabit the Orient, as their cells have been found there, and there are Oriental stories surrounding them.
Mysterious cells discovered in the Orient, transplanted into Tager. They were the subject of Kokonoe's research, but details are still unknown. What is known is that whoever they are transplanted into lose their former appearance. 
- BlazBlue: Calamity Trigger, Hakumen's story - -Lies and Truth-
- BlazBlue Material Collection, Iron Tager Short Story - Trust You
- BlazBlue: Central Fiction, Library Mode, Item #012