A Cold Day in Hell's Kitchen - S2-E13
Continuity mistake: Before the last fight there are over 30 ninjas waiting for Daredevil and Elektra. Elektra kills like 5 or 6 of them and Matt struggles just with Nobu, but when the fight is over the rest of the ninjas are dead below the water tank.
Chosen answer: After some research, I can say that no, Cass is not a character of Marvel, nor is he present in the Netflix show. As far as I can tell, it's either a real person, or a random name chosen for the coffin, most likely the former. In another season of Daredevil, the show utilized a real cemetery displaying real names in the tombstones.