Ciel sits next to 4 phonetic neighbors — Siel, Siell, and Cielo are the closest matches by sound. Most share Ciel's French-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"sky, heaven"
"meadow dweller"
"broad meadow, wide clearing"
"treasurer, keeper of treasure"
"supplanter"
"of the hawk"
"large settlement, big homestead"
"occupational name for a burner of coal or lime"
"night"
"diverse mixture"
"little king, of royal descent"
"consecrated, worthy of reverence"
"golden one (from Latin aurum, gold)"
"beauty, goodness"
"associated with the master of color"
"eager enthusiasm"
"companion of God"
"son of Gregory"
"new town"
"resembling God"
"initials — varies by family"
Similar sound: These names share phonetic patterns with Ciel — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have French origins, giving them the same cultural heritage as Ciel.
Rhythm: Ciel is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.