agent
alternate
alternative
analogy
backup
change
changed
comparison
copy
counterfeit
deputy
double
dummy
elf child
equal
equivalent
ersatz
exchange
fake
fill-in
ghost
ghostwriter
idiot
imitation
inconstant
locum tenens
makeshift
metaphor
metonymy
next best thing
oaf
oof
personnel
phony
pinch hitter
proxy
relief
replacement
representative
reserves
ringer
second string
secondary
shape-shifter
sign
simpleton
spares
stand-in
sub
substituent
child
fry
kid
minor
nestling
nipper
shaver
small fry
tiddler
tike
tyke
youngster
a-lying
creature
gecko
polypus