Regarding the qualities of a confessor, this is going to sound too obvious to be helpful but it does tell us something essential. He has to be an ordained priest with permission to hear confessions. The pardon of our sins depends not on the qualities of the individual priest but on the power of the sacrament of holy orders which is exercised through delegation by the bishop. (The technical term is faculties to hear confessions). That is what makes confession work.

However, there is another element to a confessors work and that has more to do with his individual qualities, and it consists in the gift of counsel, and the gift of tact, being able to help a needy soul to open up. You see, if a person in spiritual direction wants to hide a part of his life, or if he has sins he does not want to overcome it means the spiritual direction is not going to be effective, but the person does not commit sacrilege because of his dispositions, since spiritual direction is not a sacrament. On the other hand in confession, if the confessor realizes that the penitent is hiding something important, or says, I committed such-and-such a mortal sin but I'm not sorry, I would do it again in the same circumstances, the confessor cannot absolve him, but has to help him to acquire the proper clarity and dispositions, giving him hope and without humiliating him. So, prudence, patience, tact, real love for souls, honesty, and good doctrine are all qualities a confessor should have, generally speaking.

God bless.