"That's because Shinji hit you in the head with a rock." Byodoin replied matter of factly, starting to skillfully hook Marui onto his body board. This wasn't his first rescue mission.

One Marui was all strapped in he dug through the supplies for a backpack with a strange bracket on it... which he hooked to the body board and then casually put on.

Yes, he was wearing the body board like a backpack, with Marui still laying perfectly flat. It was probably something only he or Kin could accomplish. And then, with Marui strapped to his back, he started to steadily climb the ladder.