In Chapter 6, meet Robert then complete his "Plight" side quest.
Finn will meet you in Chapter 4. Complete his side-quest, "Finn's Plight".
Naturally occurs during the storyline, when Rubella joins Aurora's party.
At the start of Chapter 8, you will meet Oengus. Then, complete the "Plight" side quest.
At the start of Chapter 9, you will reach Piscean Village. An NPC will ask you to find her parents. Move straight and get inside the tower to solve a puzzle. Defeat the Boss to get the "Gen's Plight" side quest. Return to Piscean Village and speak with Gen.
Naturally occurs during the storyline, when Norah joins Aurora's party in Chapter 5.
At the start of Chapter 7, fly up from the large open area to reach a narrow space. You will get to Tristis at the top of the cliff. Speak with him and reunite him with his sister to complete the "Rubella's Plight" side quest.
By beating the game, you will unlock a New Game+ feature. In this, your items (barring Quests), Oculi, and Confessions will be carried to the next new game you play, as will your levels and skills once the characters can be re-recruited. However, note that your Coffers need to be reobtained, and that enemies are now stronger (but also yield more EXP.!).
Successfully complete the game. New Game+ mode features all the Oculi, Confessions, and items (except for quests) from your original playthrough, as well as your characters' levels and skills. Nolte: All previously collected Coffers will be reset and must be be found again. Enemies will be stronger but worth more experience points.