Yup.
tbh i dont think mystery guy is host because for one he very rarely fronts so that just takes off the host label from him and also i think yall are confusing the word host and the “original” (even tho theres no such thing) as the host is the alter that is outside the most which could be rei or kazuha and also i think mystery guy is more of an internal self-helper alter but thats just a theory of mine as none of the usual DID jargon used to describe alters has been used in this manga and by that i mean it hasnt been specified which alters hold which roles like protector, persecutor, fragment, etc. so honestly who knows lol also im not someone with DID or a psychiatrist so take my words with a grain of salt lmao i could be completely wrong so this is just a theory of mine
I think Mystery Guy is the host...
The way he is able to handle all the alters and talk them into merging into the host... And his face at the end. I think he is the host and he knows it