Recalcitrant
Stubbornly defiant of authority or control
Redundant
Repetitive, unnecessary
Relevant
Important to the matter at hand; pertinent
Reprehensible
Worthy of blame; disgraceful
Resolute
Firm, determined, unwavering in the face of challenges or obstacles
Reticent
Restrained, uncommunicative
Rigorous
Strict, harsh, severe
Saccharine
Overly sweet or flattering
Sage
Wise or knowledgeable, usually from experience
Scanty
Inadequate, minimal
Scrupulous
Strict or careful in ethical sense
Servile
Submissive and subservient
Skeptical
Doubting
Solemn
Grave, serious
Static
Stationary or unmoving
Stoic
Outwardly indifferent to circumstances, especially to bad or challenging times
Stringent
Strict or restrictive
Subtle
Not obvious; also, able to make fine distinctions
Supercilious
Haughty, insultingly patronizing
Superficial
On the surface only; shallow; not thorough
Taciturn
Untalkative by nature
Tangible
Touchable, not imagined or figurative
Tedious
Boring, overly detailed for no reason
Temperate
Moderate or restrained
Tenacious
Dogged or determined in the pursuit of an objective
Tentative
Temporary; uncertain; experimental
Timid
Shy, afraid of attention
Uniform
Consistent throughout; the same for everyone or in every situation
Unprecedented
Happening for the first time; never seen before
Vehement
Strong, urgent, passionate
Verbose
Wordy, overly talkative
Virulent
Malignant, malicious, full of hate
Volatile
Highly unstable, explosive
Voluminous
Very large, spacious
Willful
Deliberate, obstinate, insistent on having one’s own way
Zealous
Enthusiastically devoted to a cause or idea