I think it's pronounced reh-ee-nah (with no emphasis on any of the syllables).
― Anonymous User 7/24/2023
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There were 2 Morning Musume Members that had this name (Reina Tanaka (2003-2013 and Reina Yokoyama (2017-Present)) Tanaka Reina was a 6th Gen Member, and Yokoyama is a 13th Gen Member.