Si sits next to 4 phonetic neighbors — Cy, Sai, and Sy are the closest matches by sound. Most share Si's Chinese cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"God has heard"
"field settlement"
"iron-willed, strong"
"warrior nation"
"sun"
"God is gracious"
"from the alder grove"
"house of God (possible Bethel derivation)"
"associated with Saint Spyridon, miracle worker"
"skilled metalworker"
"Ford by the windy place; a river crossing in a breezy location"
"Dark one; descendant of Ciar"
Similar sound: These names share phonetic patterns with Si — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Hebrew origins, giving them the same cultural heritage as Si.
Rhythm: Si is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.