Strong-Willed Strong of Heart in Chinese / Japanese...

気の強い means strong-willed or strong of heart in Japanese.

Here's the character breakdown of this Japanese title:
気 (ki) spirit; mind; heart; nature; motivation; intention; feelings; essence.
の (no) possessive particle.
強い (tsuyoi) strong; powerful; mighty; potent; resistant; resilient; durable.

Note: Because this selection contains some special Japanese Hiragana characters, it should be written by a Japanese calligrapher.

意志堅強 can mean either “strong-hearted,” “strong-willed” or “determination.”

The first two characters can be translated as “will,” “willpower,” “determination,” “volition,” “intention,” or “intent.” But, it should be noted that this first part possesses the element of “heart” in the lower portion of both characters (they also partially carry the meaning “with the whole heart”).

The last two characters mean “strong” or “staunch.”

Chinese word order and grammar are a bit different than English, so in this case, they are in reverse order of English but have the correct meaning in a natural form.

See Also:  Strong Willed | Discipline | Will-Power

鉄心石腸 is a Japanese proverb that suggests you should have the inner-strength and will as hard and steadfast as iron.

It's the Japanese way of saying, “stay strong.” This is an especially uplifting thing to say to a person in distress or recovering from a disaster. It's kind of the survivor's creed.

If you literally translate this, it means “iron will, stone guts” or “iron heart, rock-hard bowels.”