Someone even-tempered stays calm as a habit. Small annoyances do not light a fire in them; they do not snap, slam doors, or swing from sweet to furious in a minute. Temper here is your usual mood, especially how easily you get angry — not the verb to temper steel, and not a bad temper on one particular afternoon. Even means steady, like a line that does not jump up and down. So an even-tempered person is not a person with no feelings. They can be sad or firm. They are just not easily thrown into anger. It is a compliment, a little old-fashioned and more common in writing or careful speech than in teen chat, where people might just say they don’t get angry easily or they have a calm personality. Compare even-handed: that is about fairness to other people; even-tempered is about the weather inside you.