Crane Keycard

Ship Deck, cage-ambush area: your partner runs ahead and gets locked in a cage; Majini pour out of a container and from above (one stays in the container throwing explosives: kill him first). One of them mutates into a Duvalia; kill it and it drops the card.

Opens the door to the crane control panel, which frees your partner.

It only exists if the trap happens. If you snipe the Majini on the crane mast from the observation tower, your partner isn't caged and the card never appears. Those two Majini also drop Topaz (Oval): it's the faster route.