St. Thomas explains what it means to call someone a good person:

He who has a will is said to be good, so far as he has a good will; because it is by our will that we employ whatever powers we may have. Hence a man is said to be good, not by his good understanding; but by his good will.

This is primarily a recognition that this is how people actually speak. When we talk about a good person, we do not mean someone who understands things well, or someone who plays the piano well, or someone who is good at driving a car, but someone who has a good will: someone who loves, wishes for, and chooses good things. But in addition to recognizing how we normally speak, St. Thomas is trying to explain why we speak this way. A good piano is a piano that functions well as a piano, and in a similar way, a good person would be someone who functions well as a person. And since the will guides all human activities, a person functions well who has a good will, and a person functions badly who has a bad will.

Pope Benedict XVI, in his encylical Spe Salvi, speaks of such conditions of the human will:

Our choice, which in the course of an entire life takes on a certain shape, can have a variety of forms. There can be people who have totally destroyed their desire for truth and readiness to love, people for whom everything has become a lie, people who have lived for hatred and have suppressed all love within themselves. This is a terrifying thought, but alarming profiles of this type can be seen in certain figures of our own history. In such people all would be beyond remedy and the destruction of good would be irrevocable: this is what we mean by the word Hell. On the other hand there can be people who are utterly pure, completely permeated by God, and thus fully open to their neighbours—people for whom communion with God even now gives direction to their entire being and whose journey towards God only brings to fulfilment what they already are.

Yet we know from experience that neither case is normal in human life. For the great majority of people—we may suppose—there remains in the depths of their being an ultimate interior openness to truth, to love, to God. In the concrete choices of life, however, it is covered over by ever new compromises with evil—much filth covers purity, but the thirst for purity remains and it still constantly re-emerges from all that is base and remains present in the soul.

The implication here is that most people are basically good at a fundamental level. They have a good will simply speaking, even if not in every respect. Some Catholics might object to Pope Benedict’s position, saying that it is not easily reconciled with previous Catholic teaching, much in the way that James Larson condemns Amoris Laetitia. If people remain fundamentally good as long as they have not “lived for hatred and have suppressed all love within themselves,” it is not easy to see how this can be reconciled with Pope John Paul II’s teaching in Veritatis Splendor when he condemns theories that separate a fundamental option from particular acts:

67. These tendencies are therefore contrary to the teaching of Scripture itself, which sees the fundamental option as a genuine choice of freedom and links that choice profoundly to particular acts. By his fundamental choice, man is capable of giving his life direction and of progressing, with the help of grace, towards his end, following God’s call. But this capacity is actually exercised in the particular choices of specific actions, through which man deliberately conforms himself to God’s will, wisdom and law. It thus needs to be stated that the so-called fundamental option, to the extent that it is distinct from a generic intention and hence one not yet determined in such a way that freedom is obligated, is always brought into play through conscious and free decisions. Precisely for this reason, it is revoked when man engages his freedom in conscious decisions to the contrary, with regard to morally grave matter.

The idea here is that if someone does something seriously wrong, even in an individual case, the person becomes a bad person simply speaking. This does not seem to fit very well with Pope Benedict’s suggestion, which seems to imply that someone can become truly evil only through a long process which eliminates love and goodness from their life.

The tension here is real. I have touched on this issue elsewhere, as for example while discussing the human tendency to divide people into “good people” and “bad people.” Nonetheless, it is presumably possible to reconcile these statements at least in a technical sense, much as I showed that Pope Francis does not contradict Catholic doctrine in Amoris Laetitia. 

In any case, common sense is enough to tell us that being a “good person” is to some extent a matter of degree. Most people care about doing good to some extent, even if some care more than others, and most people wish to avoid evil, even if they do not avoid every evil, and even if they have no wish to avoid certain particular evils.