constant
direct
arrowlike
assiduous
continuing
dead straight
diligent
dogged
enduring
even
faithful
firm
fixed
flat
flinty
horizontal
immediate
immovable
immutable
in a line
inalterable
indefatigable
indomitable
industrious
inflexible
insistent
invincible
irreversible
lasting
level
lineal
linear
loyal
never-tiring
obstinate
one-way
patient
patient as Job
permanent
perseverant
persevering
persistent
persisting
pertinacious
plodding
plugging
preoccupied
rapt
rectilineal
dauntless
firmest
self-sacrificing
single-minded
staunch
steadfast
tireless
unalterable
undeviating
undying
unfailing
unflagging
unflinching
unquestioning
unshakable
unshaken
unshrinking
untiring
whole-hearted
Irving
conserving
curving
deserving
observing
preserving
reserving
serving
undeserving
unnerving