Here is my question. Lets say I first play
Flame-Kissed (Core) as an attachment to an opponent character and then in the same phase I play
Forever Burning (Core) to the same character.The character's strenght was 3 before I play. My question is if the character is considered dead after these actions or I should have played first
Forever Burning (Core) and then
Flame-Kissed (Core) in order to kill the character

The order you play them (in the same phase) won't matter, except if your opponent chooses to interact with them or take an action in between you playing the two.

Example; (Your Action) You use Forever Burning, (His Action) he Marshalls an attachment on that character or uses a strength boost (eg Naval Escort) to bring his strength back up, (Your Action) Now using Flame-Kissed would do you no good, since the character is protected by an attachment and/or his strength is too high to be within kill range..

The "kill at 0" part of the attachment is a passive effect that will be checked for as long as the attachment is on the character. You don't have to bring the character's STR down to 0 by playing the attachment; it just needs to happen while the attachment is on the character (and active).

ok thank you both.Although my friend (House Stark) who lost an battle this way still thinks that the order matters. Anyway ''Ef Agonizeste".

ok thank you both.Although my friend (House Stark) who lost an battle this way still thinks that the order matters. Anyway ''Ef Agonizeste".

The order would matter if not for what Ktom indicated. Flame-Kissed is perpetually checking whether or not the character's strength is 0 while it is attached to a character. Once it is 0, BAM, dead character.

A non-passive effect that would care about the order would look more like this:

"Any Phase: Choose a character. Reduce that characters strength by 2. If this reduces that characters strength to 0, kill it."

I don't think there are any cards like that in the game at the moment, but this should get the idea across.