continual
implacable
intense
adamant
adamantine
assiduous
cast-iron
constant
continuing
cruel
de longue haleine
diligent
dogged
dour
endless
enduring
extended
faithful
filled out
firm
flinty
fundamentalist
grim
hard
hard-core
hidebound
immovable
immutable
impliable
inalterable
indefatigable
indomitable
industrious
inelastic
inexorable
inflexible
insistent
intransigent
invincible
iron
ironbound
ironclad
ironhanded
irreconcilable
lasting
lengthy
long
long-drawn-out
long-spun
long-winded
longiloquent
relent
unrelentingly
unrelentingness
fiendish
fierce
imperious
relentless
remorseless
tireless
unceasing
uncompromising
unforgiving
unreasoning
unremitting
unsparing
vindictive
cementing
circumventing
consenting
denting
dissenting
fermenting
inventing
lamenting
misrepresenting
presenting
preventing
reinventing
relenting
renting
repenting
representing
resenting
supplementing
tenting
venting