Sekai sits next to 3 phonetic neighbors — Kai, Seki, and Sakai are the closest matches by sound. Most share Sekai's Japanese cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"world, the whole world"
"mother"
"clear, tinkling sound"
"little king"
"healthy, wise"
"harmony, one"
"talent, ability"
"morning glory, morning well (depending on kanji)"
Similar sound: These names share phonetic patterns with Sekai — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Japanese origins, giving them the same cultural heritage as Sekai.
Rhythm: Sekai is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.