Cordie is perhaps the most human looking monster that we've seen so far in the pilot. Should the series continue, we will eventually discover that she has a human parent and an arachnoid one. Cliffside being known to be a town constantly surrounded by monsters, who's to say that a few townsfolk eventually fell in love with one or two of these supernatural creatures and conceived a halfbreed?
Cordie was Originally Human
Cliffside is a mystical place, it wouldn't be a surprise that some unfortunate townsfolk found themselves cursed or afflicted with something that permanently changed them
. Cordie could very well be one of these people. Wendigos in myth are former humans who transformed after succumbing to cannibalism so it wouldn't be a surprise that other altering curses are present in the outskirts of town.
Cordie was Originally a Regular Spider
A converse of the above examples, Cordie was your average forest variety spider before being hit by the town's curse which transformed her into a humanoid spider. Alternatively this can go on a fairy tale route which involves a spider falling in love with a human so bad that she wished to become human just like him so that she can be with him as a lover. At some point things didn't turned out too well and stopped her from becoming fully human (due breaking a condition of the wish and other mishaps) which ended with her half transformed between a human and a spider. This naturally scared the human away and crushed her dreams. This will also explain why she seemingly instantly fell madly in love (or at the very least a big crush) on Waylon.
It's a pretty common trope for monstrous humanoids to have lengthy lifespans so who's to say that Cordie possessed this quality as well? Her interactions with Yannis and Death Itself implies that they've known each other for some time now. She also revealed that she understands Death to a degree but not ny much because she is a bit "rusty". Implying that she understood its language enough at some point in the past. Death Itself is understandably a very ancient entity so this also implies that that point in the past is a very long time than just a few years.
Yannis will Eventually be Resurrected at Some Point
Being friends with Death Itself does have some benefits and it may eventually decide to bring its dead friend back to life at some point in the series. Yannis seems to have visibly experienced this before as well. She appears as a long extinct prehistoric animal with ghoulish features (dark sunken eyes and a pale, corpse-like coloring) implies that Death has resurrected her multiple times already.
Waylon is a Distant Ancestor of Marco Diaz
It's pretty obvious this would pop up in here eventually.