Killua sits next to 3 phonetic neighbors — Silas, Killian, and Kaito are the closest matches by sound. Most share Killua's Japanese cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"fictional invented name, no established meaning"
"son of the devotee of John"
"griffin (mythical creature symbolizing strength)"
"faithful, loyal"
"God will uplift, God will exalt"
"dear one"
"night watchman"
"clearing in the forest"
"people's power"
"little swift one, bright and pleasant"
"creative American name combining Ty + Reese"
"oak, fertile, fruitful"
"son of Dierk (patronymic)"
"comfort, ease, happiness"
"a days travel; a trip"
"belonging to God"
"heath dweller, one from the heathlands"
"Dane's settlement"
"cheerful, merry"
"firebrand, bold warrior"
"beloved son"
Similar sound: These names share phonetic patterns with Killua — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Japanese (fictional) origins, giving them the same cultural heritage as Killua.
Rhythm: Killua is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.