Here is the full list of all 11 Under One Wing achievements.
Welcome to Under One Wing!
Clear the Common Route
Clear Mimari's Route
Clear Kazusa's Route
Clear Nahoko's Route
Clear Hikari's Route
Saw the fight postponed for another day.
Gave it everything you had, but in the end...
Saw the end of a sibling rivalry.
Helped settle the score with an old rival.
Made things crystal clear to your "rival."