adjectiveadverb

treu

disloyal/unfaithful: untreu , illoyal
unreliable/inconstant: unzuverlässig , wankelmütig , wechselhaft

faithful · loyal

Jemand, der zuverlässig zu einer Person oder Sache steht und nicht wechselt oder verlässt.

treu means faithful, loyal, true, or devoted. It describes someone or something that remains steadfast, reliable, and committed.

Common uses:

loyal to a person/group: jemandem treu sein = to be loyal/faithful to someone
faithful in a relationship: seinem Partner treu sein = to be faithful to one’s partner
true to principles/values: seinen Idealen treu bleiben = to stay true to one’s ideals
faithful/accurate reproduction (less personal): originalgetreu = true to the original

treu is a basic adjective (not transparently composed of smaller parts in modern German).
Related formations:
- Treue (noun) = loyalty, faithfulness
- getreu (adjective/adverb) = faithful; true to (often in fixed phrases like getreu dem Motto…)
- compounds like werkgetreu, originalgetreu = faithful to the work/original

Depending on context:

loyal/faithful (personal loyalty): loyal, ergeben, zuverlässig, standhaft
faithful in a relationship: monogam (more technical), verlässlich (partly)
true to something (principles/identity): konsequent, beständig, wahrhaftig (more “truthful” than “loyal”)