Riku sits next to 4 phonetic neighbors — Nico, Rico, and Riku are the closest matches by sound. Most share Riku's Japanese cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"strength, power"
"luminous second son"
"happy, joyful, blissful"
"shrine maiden"
"authority, firm determination"
"warrior"
"Dracula spelled backwards (fictional)"
"precious, valuable"
"thunder and lightning"
Similar sound: These names share phonetic patterns with Riku — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Japanese origins, giving them the same cultural heritage as Riku.
Rhythm: Riku is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.