Rupert sits next to 4 phonetic neighbors — Robert, Hubert, and Herbert are the closest matches by sound. Most share Rupert's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"bright fame"
"industrious ruler"
"sea, ocean (via Kai)"
"circular stone fortress"
"teacher, learned one"
"symbol of resilience and mourning"
"lapis lazuli (the stone)"
"powerful giant, cosmic force"
"fighter, battle"
"one who tills the land, farmer"
"pleasant stream"
"warrior in the field"
"gift of Isis"
"gift of God"
"he to whom it belongs"
"brave in battle (via Casey etymology)"
"white, fair-complexioned"
"anointed one"
"fire, flame"
"peace (folk etymology)"
"modern invented name with energetic sound"
Similar sound: These names share phonetic patterns with Rupert — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Germanic origins, giving them the same cultural heritage as Rupert.
Rhythm: Rupert is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.