A good-tempered person is pleasant and kind, and it takes a lot to make them angry. Temper here is a person’s usual mood — the same idea as in bad-tempered, which means quick to get angry. So the word is really “having a good temper”, not “made well” like tempered steel. It sits very close to good-natured: good-natured talks a little more about kindness, good-tempered a little more about not losing your temper. You can use it of people, and sometimes of animals that stay calm with strangers.