中堅【ちゅうけん】
nucleus, backbone, mainstay, key figure, medium-level, mid-level, middle-ranking, midsize, main body (of troops), crack troops, select troops, center field, centre field, center fielder, centre fielder, athlete competing in the middle-number match in a team competition, i.e. second in 3-on-3, third in 5-on-5 (kendo, judo, etc.)
堅持【けんじ】
holding on to, sticking to
堅実【けんじつ】
steady, sound, stable, safe (e.g. business), reliable, trustworthy, solid
手堅い【てがたい】
sound, sure, safe, secure, trustworthy, reliable, solid, steady, firm
堅固【けんご】
solid, strong, firm
堅苦しい【かたくるしい】
formal, strict, ceremonious, stiff