Does Celador leaves the staging area somehow or he must stay there for as long as he is in theg ame ?

While Celador is in the staging area, he is committed to the current quest.
Forced: After players quest unsuccessfully or a character leaves play, deal 1 damage to Celador.
If Celador leaves play, remove him from the game

Hon Manual P3

If an effect allows the players to take control of any of
these objective ally cards, it is moved into the controlling
player’s play are

thx ... is there any card with which I can bring Celador under my control ?

If Celador is still alive when you finish quest card 1, he will join you. The card text on quest card 1 tells you to bypass quest card 2 and go to card 3, which looks like this:

ok, thx