There's something wonderfully nuanced about describing seriousness in English. Beyond the basic 'serious', words like 'earnest' carry a warmth that suggests genuine intent without stiffness. 'Diligent' leans into the persistent aspect, while 'conscientious' implies thoughtful attention to detail.
For a more poetic touch, 'steadfast' conveys unwavering dedication, and 'assiduous' adds a layer of refined effort. In creative contexts, I've seen 'intentional' used beautifully to describe purposeful seriousness—like in 'NieR:Automata's' 2B, whose actions feel deeply intentional despite her stoicism.