Is hearing news related to someone's character, without delving into it or commenting on it, considered backbiting and an affront to people's honor, and if so, what should the listener do to atone for this sin?

You must advise the one who backbites his brother with wisdom and explain the danger of backbiting, and defend your brother's honor, because backbiting is forbidden by Sharia. The backbiter is like one who eats the flesh of his dead brother. The Prophet, peace and blessings be upon him, likened backbiting to mentioning your brother with what he dislikes. If what you say is true about him, then you have backbitten him; and if it is not true, then you have slandered him. You must repent to Allah and seek His forgiveness if you have fallen short in defending your brother's honor.