Ours is the Fury doesn't care about challenge icons or stealth. Those requirements are only checked during the "normal way" to add characters to a challenge, during the"declare attackers" and "declare defenders" frameworks, whereas this ability is an action performed during an action window within a challenge.

For example, Robert Baratheon can be made to participate in an intrigue challenge with Ours is the Fury's ability.

Likewise, if Robert is stealthed, he can still be added as a defender with Ours is the Fury (provided he was already kneeling).