Salve,
First - just to check if I understand correctly, The stealth keyword means you can choose a character whithout stealth keyword, who can not defend against the attack. If you have for example three characters with stealth, you can choose three character on the opponents side who can`t defend?
Second, if he/she choose Greatjon Umber not to defend, because of the stealth keyword , can Greatjon Umber defend if he is kneeling because of his ability: "Challenges: Kneel Greatjon Umber to have him participate in the current millitary or power challenge as either the attacker or as the defender"? I mean he cant because of the stealth, but he can, because of his ability. Which one wins? The stealth or the ability?