Pesah - Haggada
Maggid - 4 sons
We list 4 sons, each with his style of discussion about Pesah. The wise son, the wicked son, the simple son and the son who can't even ask a question.
Out of all these kids, one style seems to be missing. The righteous son. The tzaddik.
The wise son and the wicked son are not opposites. The wise son is an intellectual. Nothing to do with his level of piety.
We even see further that the wise son is not necessarily a tzaddik, as he asks, "What are these rules that Hashem commanded you" - similar to the wicked son, he asks about you rather than including himself. There is some sense of dissociation. It is more of an intellectual pursuit for him.
I have heard from rebbeim a number of times that a person can learn Torah all day long and not think once about Hashem. We do things by rote. We learn because we are supposed to, or even because we love to, but we do not necessrily relate to Hashem on a daily basis.
The opposite of the wicked son would be the tzaddik son, the righteous son. Why is such a child not listed among the other sons? Where is the "אחד צדיק"?
A Tzaddik is not bothered by the questions, even if he has any. He lives by his faith. We know the passuk tells us צדיק באמונתו יחיה - a tzaddik lives by his faith. He has emuna, faith, in Hashem and that he is doing the right thing. the rest does not matter.
That is why we answer the wise son with a very technical answer. he asked about the rules and we explain the rules to him. An answer based on emuna would not move him. He is technical and intellectual in nature and he is looking for that type of answer.
חנוך לנער על פי דרכו - educate each child in the way that is fitting for him.
The wise son asks detailed questions, give him detailed answers. The tzaddik son has no questions, or is not looking for answers. So he does not need to be mentioned.