Multiplicity

Not necessarily even cases like hers. More like people who give names to the different sides of their personality or use mental constructs to work out internal conflicts. Or people who have voices in their head that they talk to and make decisions with but the voices don't correspond to people or even parts of themself. Or even the soulbonders whose soulbonds are just aspects of their imagination but they treat the characters as though they were real. None of those people could even remotely be considered multiple but in some cases they might find the label of single inappropriate for them.