Why Are Vanitas No Carte And Owari No Seraph So Similar????

Why are Vanitas no Carte and Owari no Seraph so similar????

So, like any idiot who likes vampire manga/anime/any media, I watched/read The Case Study of Vanitas but that wasn't enough so I watched/read Seraph of the End and it's always itched my brain with how similar they are. Are they both based on some other story or something? Because a lot of the character archetypes are the same and in the same places. First we got Mikaela and Mikhail which I mean the similarities are already there between the names and they both have lighter color palettes, and then they both have a "family" character with a darker color palette (for Mika it's more of a found family thing in the same way as TOH because I mean. Him and Yu are so fucking gay) that goes apeshit. And THEN there's the archetype of ageless god figure who is visibly feminine but also described as a man (and in The Case Study of Vanitas Luna is described as a "he" in the beginning, referred to by Mikhail as "Father," and referred to by Vanitas as a "woman" but uses they/them pronouns) and then they are also widely blamed for the existence of the main problem of the series (malnomens in Vanitas no Carte and demons/horsemen/angels/vampires in Owari no Seraph) I know it's really just a couple archetypes but they feel so similar and if anybody can tell me if they're related somehow (like based on a different story) I would be really thankful