Diece is a fan-favorite character in the Breath of Fire series, and for good reason. Her over-the-top personality and haughty interactions with other characters who have no idea what the hell her problem is add an abundant amount of comic relief to the dialogue, and her powerful attack magic makes her a fantastic addition to your active party. Her presence is usually important to furthering the overall progress of the story as well, which gives her a bangin' in-game resume that's hard to beat. This is a spoiler heavy article.
Breath of Fire
Diece's background as a legendary sorceress who originally helped seal away Myria is introduced in the first game, so her role is crucial to the back story as much as it is necessary to advance important plot points throughout the game. Once you obtain Diece she'll become a staple in nearly every event; furthering scenes with her knowledge of the ages, or just blowing shit up with magic she hasn't used in ages. Diece joins you late in the game, but with her arsenal of ranged attack magic and low TNLs she'll more than likely be a preferred addition to your active party till the end.
Breath of Fire 2
Diece returns to the second game as a secret character, reprising her legacy as an immortal sorceress. She spends the first part of the story eavesdropping at the Moto Magic Academy under the guise of a normal student, having seemingly taken an interest in its teachings of modern magic. She'll be at level 35 and already have most of the spells that she'll be able to learn when she joins your party, so she's actually meant to be an added perk to make battles easier as much as she is a nice surprise for those who enjoyed her in the first game. Unfortunately, she doesn't have any kind of role in furthering the storyline, but you can always be sure to get a laugh when you speak to her at the Hideout after completing an event (or by visiting her in her room when she isn't in your active party).
Breath of Fire 3
In BoF3, Diece is the one who informs Ryuu and Co. of Myria's involvement in the genocide of the Dragon race and her ongoing hunt for any of its descendants. If you play your cards right, Diece will take the role of a Master in the gameplay rather than joining your battle party, but her role is more crucial to the storyline than anything else. In this game it's also revealed that Diece is Myria's sister, but disapproves of her views and actions towards the Dragon race.
Breath of Fire 4
Diece is portrayed in a very interesting way in BOF4. Instead of joining your party as herself, she joins as a spiritual essence residing inside of an odd little suit of arms. The armor, called MASTER, joins your party early on, though it isn't immediately revealed that Diece is inside. Once discovered, Diece continues to provide comic relief and helps the story advance by assisting Ryuu with her knowledge as an immortal. She'll remain with your party (albeit inside the armor) till the end.