I' ve got some questions and hope that you can help me.

1. Is it possible to declare stealth to a character which has not the icon of the challenge?

2. If a character is declared as attacker or defender and loses the icon of the present challenge afterwards, can he still participate in that challenge or is he removed?

Thanks for your help.

Strossus said:

"For each of your attacking characters with the
'Stealth' keyword, you may choose, before defenders
are declared, a character without Stealth
on the defending side. That character may not
defend during this challenge."

Nothing in there about it being an eligible defender. So sure, you can choose a character controlled by the defending player that does not have the appropriate challenge icon. It's almost always meaningless to Stealth past a character who is already not eligible to defend in the challenge, but you can do it.

(And just to preempt the obligatory reference to Lost Oasis someone will make to the answer here, yes, the usual reason to stealth past an ineligible defender is because you have some sort of Response to being bypassed by stealth.)

Strossus said:

Thank you for the fast answers