indefatigable (adj.)

• abiding
• ardent
• assiduous
• constant
• continuing
• continuous
• determined
• diligent
• dogged
• enduring
• energetic
• faithful
• fervent
• hard
• hardworking
• immutable
• inalterable
• indomitable
• industrious
• inexhaustible
• insistent
• invincible
• laborious
• lasting
• loyal
• obstinate
• painstaking
• patient
• permanent
• persevering
• persistent
• persisting
• pertinacious
• plodding
• preoccupied
• rapt
• relentless
• remaining
• resolute
• sedulous
• single-minded
• sleepless
• stable
• steadfast
• steady
• sticking
• strenuous
• stubborn
• sustained
• tenacious
• tireless
• unceasing
• unconquerable
• undaunted
• undiscouraged
• undying
• unfailing
• unfaltering
• unflagging
• unflinching
• uninterrupted
• unrelenting
• unremitting
• unsleeping
• unsparing
• unswerving
• untiring
• unwavering
• unwearied
• unwearying
• vehement
• vigorous
• zealous