Continuity mistake: When Blake Tower is talking with Foggy in his car, when Tower says "You need help, Nelson", there are raindrops along the interior of the driver side door. The shot cuts to Foggy, and when it cuts back to Tower, the raindrops are suddenly more faded. (00:14:00)
Continuity mistake: When Matt and Foggy are talking with Nadeem after he testifies, in some shots, Matt is holding onto the top of his cane, but in other shots, his hand is further down. (00:39:35)
Blindsided - S3-E4
Continuity mistake: As Marcy says "the more public, the more you're protected", her hands are flat on Foggy's chest. Cut to a shot from the side and her hands are clasped in front of her by his stomach.
Resurrection - S3-E1
Continuity mistake: When Matt is recovering in the orphanage, his bandages around his torso keep changing position. They go from just barely below his nipples, to covering them completely, to way below them in the same scene.
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.