Quenya 

vórima

adjective. faithful, **(lit.) able to endure, *[ᴹQ.] continuous, *enduring, *repeated, *[ᴱQ.] everlasting
Quenya [UT/317] Group: Eldamo. Published by

voronda

adjective. faithful, steadfast (in allegiance, in keeping oath or promise)
Quenya [UT/305; UT/317] Group: Eldamo. Published by

Sindarin 

sador

noun. faithful one

Sador was Túrin's faithful servant. The meaning of this noun is deduced from sadron , assuming that these words are in the same kind of relation as hador and hadron

Sindarin [Sador (name)] Group: Hiswelókë's Sindarin Dictionary. Published by

sadron

noun. faithful one
Sindarin [UT/431] Group: Hiswelókë's Sindarin Dictionary. Published by

Noldorin 

bór

masculine name. Faithful
Noldorin [Ety/BOR; LR/147; LR/291; LRI/Bor; RSI/Bor; SMI/Bor] Group: Eldamo. Published by

Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!

Undetermined

avaltiri

collective name. Faithful

An early name of the Númenóreans who remained faithful to the Valar, later called the “Elf-friends”: Ad. Nimruzîrim, Q. Elendili. It is not clear what language this word belongs to, but it may contain the word Adûnaic word for the Valar: Avalô.

Undetermined [SD/347; SDI2/Avaltiri] Group: Eldamo. Published by

Primitive elvish

satar

root. faithful, trust, loyal, rely, steadfast
Primitive elvish [PE17/145; PE17/183] Group: Eldamo. Published by

Qenya 

voronda

adjective. faithful
Qenya [LT1A/Bronweg; QL/102] Group: Eldamo. Published by