Brain sits next to 4 phonetic neighbors — Brian, Bran, and Braid are the closest matches by sound. Most share Brain's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"noble, high"
"of the highest class or quality"
"from the hall or manor house"
"cold, icy in manner"
"narrow strait, channel"
"possessing unlimited power"
"victory of the people"
"Open land, wide expanse; one who ranges freely"
"twin (variant of Tavis/Thomas)"
"one who lives near a spring or who springs"
"dear one, beloved by the pool"
Similar sound: These names share phonetic patterns with Brain — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Celtic origins, giving them the same cultural heritage as Brain.
Rhythm: Brain is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.