unfailing (adj.)

• abiding
• assiduous
• calculable
• certain
• changeless
• consistent
• constant
• continuing
• dependable
• devoted
• diligent
• dogged
• durable
• enduring
• faithful
• fast
• firm
• fixed
• frozen
• immobile
• immutable
• inalterable
• indefatigable
• indomitable
• industrious
• insistent
• intact
• invariable
• invincible
• inviolate
• lasting
• loyal
• obstinate
• patient
• permanent
• perpetual
• persevering
• persistent
• persisting
• pertinacious
• plodding
• predictable
• preoccupied
• quiescent
• rapt
• relentless
• reliable
• remaining
• resolute
• rigid
• secure
• sedulous
• single-minded
• sleepless
• solid
• sound
• stable
• static
• stationary
• staunch
• steadfast
• steady
• stubborn
• substantial
• sure
• sustained
• tenacious
• tireless
• torpid
• true
• true-blue
• trustworthy
• trusty
• unaltered
• unchangeable
• unchanged
• unchanging
• unchecked
• unconquerable
• undaunted
• undiscouraged
• unfading
• unfaltering
• unflagging
• unflinching
• uninterrupted
• unrelenting
• unremitting
• unsleeping
• unswerving
• untiring
• unvaried
• unvarying
• unwavering
• unwearied
• unwearying
• well-founded
• well-grounded