Hagfish are primitive, eel-like fish. Kind of. In some way, each of those 3 descriptors are a lie.

Are they primitive?
They lack a spine, a jaw, a stomach, they have extremely simple eyes, no epidermis, and so on; they branched off from the other Chordates around the time that the vertebrates appeared! But they have evolved for as long as any other modern animal, and they have an extremely advanced skill for producing slime.

Are they eel-like?
Well, they have a long body with none of the normal fins we'd expect from a fish (besides a caudal fin), so visually they are kind of similar, but beyond looks they are not very comparable. For one, there are all the weird features previously mentioned, but additionally they are also only distantly related from a genetic, taxonomic perspective.

Are they fish?
Hmmmm kinda. They have no scales, almost no fins, no swim bladder - the kind of features we usually associate with a fish. But like, they kind of are. There's no formal, well-defined definition of a fish. So idk man. They live in the sea.