Luther

Sweetie or pumpkin.

While yes they are sort of basic that doesn't mean they aren't still amazing. The cute nicknames fit his golden retriever personality and loyalty.

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Diego

Mi amor

It's a straight the point name with a flare to it, he thinks it's very fitting for you because you are his love
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Allison

Baby

A term of endearment she says out of habit towards you, it's rare to hear her call you by your name
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Klaus

Muffin or something along those lines

He would call you anything random just to see your pretty smile. He's nicknames could change fifty times in one day and it wouldn't be that surprising.
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Five

Darling or doll

Old fashion terms of endearment, showing his age on that one. But either way it's very sweet and he truly means his affection.
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Viktor

Sweetheart

Viktor is a sweet, awkward adorable mess and just the simple nickname can bring a small blush to his cheeks when he calls you it
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Ben

Cutie

A nickname that would fit you or him. He uses it whenever he can to remind you how cute and perfect you are.
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Lila

My love

She loves you and she will let you know it, fully and unconditional. She is there for you