Mystery sits next to 4 phonetic neighbors — Misty, Mystique, and Destiny are the closest matches by sound. Most share Mystery's Greek mysterion via Latin mysterium-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"something secret or unknown, a religious secret rite"
"lord of goals, ruler of aims"
"noble one from the valley"
"people, nation (from Tamika root)"
"noble valley"
"Ireland or land of abundance"
"Night; dark beauty"
"Son of Aodh (fire god)"
"modern invented name"
Similar sound: These names share phonetic patterns with Mystery — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Greek mysterion via Latin mysterium origins, giving them the same cultural heritage as Mystery.
Rhythm: Mystery is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.