Hour of Darkness(PS2)/Afternoon of Darkness (PSP)/A brighter Darkness (PS3)?
I know that the PSP title lets you play as Etna...so that would be the most you get of her, i guess. --- Playing now: PSP2/PSP; Disgaea 4/PS3; League of Legends, Sacred - Underworld/PC Ahri, Lux, Morgana, Nidalee...my harem will grow!
Somewhat has a role in Disgaea 2, or so I've heard.
She's just a mere postgame cameo in 3 and 4.
Then there's the (very) soon to be released Disgaea Dimension 2. Being a direct sequel of the first game. --- This has been another (probably) uselessly informative post, by me. Booyah! http://www.youtube.com/user/MrEvilbyte
In Disgaea 2, Etna appears multiple times through the story. There's one time where you forced to lose to her since she fights you early in the game and she's Level 1000. She also joins your party somewhere near the end of the game and there's some cutscenes where she interacts with Laharl and Flonne. --- My Youtube Channel: http://www.youtube.com/user/TheDragoniteChannel/featured
For starters, she is cruising the universe in a toy spaceship in search for some candy. She introduces herself as the "Mighty Space Pirate Magical Sweet Hyperdimensional Etna". No joke.
And Flonne looks like a lifeless doll who is there to say things like "the best thing on life is to spend the day locked in your room, with all your figurines and reading manga! That's so awesome!" --- Your childhood was good, but not objectively better than others'.