A betrothed is the person somebody has promised to marry. His betrothed, meet her betrothed, the betrothed pair.
The word does not change for a man or a woman, which is why it is handier than fiance and fiancee with their French endings and accents. It is old-fashioned and literary, and nobody uses it of themselves in ordinary speech.
1nounbetrothedAn old-fashioned, literary word for the person one has promised to marry, used unchanged for a man or a woman rather than the French-derived pair.
“He presented his betrothed to an aunt who disapproved of them both.”
betrothed is a person.
Betroth is be- plus troth, an old form of truth: to pledge one's truth to another.
From Middle English -, equivalent to betroth + -ed.
Etymology from Wiktionary, CC BY-SA