Summon Night: Twin Age
centers on two siblings, Aldo and Reiha, whose adventures begin when they set out to investigate why the usually tame spirits on their island home have suddenly gone wild. Their quest soon takes them to the human world where they discover that even the humans are having their own spirit troubles. Although the story isn't overly complicated or very deep, it is good enough that you'll want to keep playing, mostly to see what impact your decisions will have on how the plot develops.
At various times in the story, you'll have the chance to decide which way the story should go. Usually you'll have to decide which part of the story to reveal next in a conversation, though other times you'll have to decide which path to take or who to believe. Deciding who to listen to plays a major part in the gameplay; the more a party member likes you, the more useful they are in battle. Slighting a character's opinions won't cause them to leave your party or stop fighting, but they won't be as effective in battle. You can also bond with characters through short conversations that bookend each chapter.
While I would stop at making these sort of conversations a major gameplay component, it would have been great if they mattered a little more. The system does a good job at pulling you into the story and making you think about what is being said, though the consequences of certain choices aren't that major. You always have direct control over the main characters and chances are you'll find one extra party member you like and stick with them for the duration of the game - so it really doesn't matter if the other people like you or not. Also, story elements always come out during conversations, even if you don't ask about them, so other than altering the ending, conversation paths usually lead to the exact same information.
Aside from going through the main quest, you can also take on several side-quests that usually yield new equipment or other treasures. You can also create monsters to join your party or create new weapons. Both add just a little more to do in the game, though I never found them to be that deep of an addition. Monster creation is limited and, with one or two exceptions, the most expensive weapon to create is usually the best one.