の人だった

was the ... person

→ の

phrase Common #5,173

was the ... person の connects a description to 人 (person); だった means “was.”

JA: 彼女は私が会った最初の人だった。

EN: She was the first person I met.

They had flashy hair, after all.

— Demon Slayer -Kimetsu no Yaiba- The Movie: Mugen Train (2020)