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?
Yes. Just before defenders are declared, the attacker can choose a number of characters controlled by the defender (that do not have the Stealth keyword) equal to the number of attacking characters with the Stealth keyword. Those characters cannot defend in that challenge.
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?
Couple of things:
First, let's make sure you're reading Greatjon correctly. He doesn't get to use his ability to participate as an attacker or defender because he is kneeling. He has to successfully kneel (ie, go from standing to kneeling) in order to pay the cost of his effect that makes him an attacker or defender. So your comment "defend if he is kneeling" is a little off (or not quite what you mean to say). The question here is whether he can defend when he is Stealthed by triggering/using his ability (which he has to go from standing to kneeling to pay for).
Second, Greatjon's ability lets him become a defender (ie, defend in the challenge) without being declared. Most of the time, this distinction is enough to let him use his ability because the limitation is usually on being declared (eg, Joust says you can only declare one defender; but Greatjon isn't declared, so you can get a second defender in there, no sweat). However, Stealth says that the character "cannot defend," which is much broader than "cannot be declared as a defender. So Stealth can stop him from using his ability to defend.
This is because (third) because the word "cannot" always "wins." The word "cannot" is considered the absolute in this game. So in most cases, "cannot" should effectively be read as "don't even try." If Greatjon "cannot defend," he is effectively prevented from even triggering his ability in an attempt to defend.
However (finally), remember the timing. Greatjon's ability is a Challenge ability. It can be used in two different places: 1) After attackers are declared but before defenders are declared, and 2) After defenders are declared but before the challenge is resolved. Stealth happens just before #2 starts, which means the "cannot defend" limitation would stop Greatjon from using his ability to become a defender at #2 (after Stealth is applied). So all you have to do with Greatjon is have the sense to use his ability in #1, before the attacker has the possibility of assigning Stealth. That is because once a character is defending, nothing removes him from the challenge - short of an effect that specifically removes him from the challenge or from play.
So in the end, Stealth can stop Greatjon from using his ability to defend, but someone who is paying attention can use the timing of Stealth to his advantage and get him into the challenge anyway.