SpoilerMarcel was a comrade and childhood friend of Reiner Braun, Bertolt Hoover, and Annie Leonhart. He was part of the group sent by Marley to attack the Eldians of Paradis in year 845, but his mission was cut short by the sudden appearance of Ymir in her mindless Titan form.
(Source: Shingeki no Kyojin Wikia)
