Wardens of the North 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" framework steps, whereas this ability is an action performed during an action window within a challenge.
For example, Catelyn Stark can be made to participate in a military challenge with this ability (provided you already have a participating Stark character).
Likewise, if a character is stealthed, it can still be added as a defender with the Wardens of the North ability.
Note that you can NOT kneel an opponent's Stark character to have it participate on your side, because "kneel another Stark character" is the cost to use the ability, and costs can only be paid with cards you control.