I think Natsu and Lucy should have a girl called Nashi and  a boy called Luke. Have a look.

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Name: Nashi Dragneel

Gender: Female

Magic: Fire dragon slayer

Hair color: Pink/Salmon like Natsu's

Eye color: Brown like Lucy's
Clothes: Similar to Natsu's

Hair: Long, out, a bit messy

Habits: Fighting with Rain

Likes: Storm, Gale, Nora
Dislikes:Rain, Nova

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Name: Luke Dragneel

Gender: Male

Magic: Celestial spirits

Hair colour: Blonde like Lucy's
Eye color: Onyx like Natsu's
Clothes: similar to Lucy's first outfit

Hair: Short, well brushed, neatly cut

Personality: Like Lucy

Habits: Daydreaming

Dislikes: Getting cold, blueberries

Likes: Strawberries, writing his novel, reading