Another word for "serious" can depend on the context, but some common synonyms include earnest, grave, critical, important, solemn, and significant. Other options depending on nuance and usage are severe, profound, weighty, thoughtful, and sober. For example, in a context of being thoughtful or sincere, "earnest" or "solemn" fits well; in a context of importance or criticality, "grave," "critical," or "momentous" might be better choices.
