Just a quick question, where was this part confirmed? "Rei often gets manipulated by Rin for some reason."?

I haven't seen this info yet in the anime... is it in the official site?

--AdventureWriter28 (talk)

It stood on his profile on the official site. Anime News Network has translated it in English.
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I'm not sure how Amine News Network translated it since I can't find such a page, but the official website definitely says that it's Nagisa who manipulates him. "何故がに振り回されていることが多い。" (taken from the official site, bolding mine). 18:25, July 25, 2013 (UTC)

Rei Ryugazaki's English Voice

FUNimation has just announced the voice of Rei from the dub of Free! Eternal Summer and he will be voiced by J. Michael Tatum, if you want to see the proof click here. Animefan850 (talk) 20:08, November 11, 2014 (UTC)