Similar word to seriousminded:

allegiant, ardent, calm, constant, cool, dependable, equable, fast, imperturbable, intense, levelheaded, liege, loyal, poised, reliable, reserved, resolute, sedate, self-possessed, sensible, serene, serious-minded, settled, single-minded, sober, staid, staunch, steadfast, unswerving, unwavering, wholehearted