"an Unsullied" should be accepted too, no? Can't we change the article at least to say it's one unspecified Unsullied? Would Grey Worm have to say "I am the Unsullied" (fixed expression) rather than simply "I'm Unsullied"? I'd have guessed the latter to be fine (always with a capital "U", ofc).
"The girl sees an Unsullied" is not accepted because the ending is -ī which indicates the accusative plural. The ending for the accusative singular is -i.
- Riña Dovaogēdi urnes (note the short -i at the end) – "The girl sees an Unsullied"
- Dovaogēdy iksan – "I am an Unsullied"
- Dovaogēdi iksi – "We are (the) Unsullied"