der Retter
rescuer
A person who frees another person or a thing from immediate danger or distress.
Der Feuerwehrmann war der Retter des kleinen Jungen aus dem brennenden Haus.
The firefighter was the rescuer of the little boy from the burning house.

rescuer / savior
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rescuer
A person who frees another person or a thing from immediate danger or distress.
Der Feuerwehrmann war der Retter des kleinen Jungen aus dem brennenden Haus.
The firefighter was the rescuer of the little boy from the burning house.

'Der Retter' (the rescuer/savior) is derived from the verb 'retten' (to rescue/save). The ending '-er' forms a noun here that describes the person who performs the action of the verb.
Wer jemanden aus einer Gefahr rettet, wird zum Retter.
Someone who rescues someone from danger becomes a rescuer.
'Retter' (rescuer) is often combined with professions or organizations that provide help, for example 'Bergretter' (mountain rescuer), 'Seenotretter' (sea rescuer), or 'Rettungsschwimmer' (lifeguard). A fixed expression is 'der Retter in der Not' (the rescuer in a time of need).
Die Bergretter konnten die Wanderer sicher ins Tal bringen.
The mountain rescuers were able to bring the hikers safely down into the valley.
In everyday language, 'Retter' (savior) is often used in an exaggerated and humorous way for someone who helps with a small problem. The exclamation 'Du bist mein Retter!' (You're my savior!) is very common.
Danke, dass du mir den Stift leihst, du bist mein Retter!
Thanks for lending me the pen; you're my savior!
In a religious context, 'der Retter' (the Savior) is a fixed term and usually refers to Jesus Christ. You can recognize this meaning from topics like faith, church, or the Bible.
In vielen Predigten wird Jesus als der Retter der Menschheit bezeichnet.
In many sermons, Jesus is referred to as the Savior of humanity.
hero
'der Held' and 'der Retter' can both describe a person who helps others and is admired for it. 'der Retter' is more specific when someone saves a person or thing from danger or serious distress. 'der Held' is broader: a hero may rescue someone, but they may also stand out through courage, strength, or an important good deed. The words can be confused because a rescuer is often seen as a hero. In practice: if the act of rescuing is the main point, 'Retter' fits better; if admiration or courage is important, 'Held' fits better.

Der Retter zog den Jungen aus dem kalten Wasser.
The rescuer pulled the boy out of the cold water.
Nach seinem mutigen Einsatz galt der Feuerwehrmann für viele Kinder als Held.
After his brave actions, many children considered the firefighter a hero.