I'm sure Tanjirou won't be able to do all 12 forms by himself so maybe Nezuko will show up and complete the other half of the forms and together they'll form the 13th form... unless Nezuko turns back to human before then
More like nezuko would show up nowhere and kick right into his head(づ￣ ³￣)づ
The "merge" thing was accurate since alters don't disappear... and happy triggers CAN bring out alters like what happened in this chapter :0 I know this is just fiction but it's really important to be properly informed about something as misrepresented as DID. I recommend DissociaDID on youtube if you want to learn more about it! They make educational videos as a person with DID