決め手の勇士 by きょーすけ
"The Deciding Warrior"
A lot of meta has been written about the meaning of Reiner’s name. It’s derived from Raganhar, which means “counselor/advisor” + “warrior.” Reiner’s name meaning is often condensed down to simply “deciding warrior.” When translating this information into Japanese, however, artist Kyosuke chose to write 決め手の勇士 kimete no yuushi which means “THE deciding warrior” in the sense of, like, the trump card. The ace up the sleeve. The person who clinches the victory.
I think that is a perfect translation on a lot of levels.