https://www.doujinshi.org/book/504836/

Set in the interval between Galuf leaving and the remaining party going to World 2, this doujin starts off with Bartz realizing he's in love with both Lenna and Faris. While he chews himself out for being such a dick, Lenna has no such compunctions. For this is Mabo Lenna, she of the abnormally short skirt and insatiable sex drive. Nothing will stop her from her dream threesome.

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This is Mabo Udon Teishoku's first FFV work, and it sets the stage for most of the others. Unless otherwise noted you can probably assume they all take place in the Lenna x Bartz x Faris universe.

Part 2: /g/520621/58e2e01b9a/
In which Lenna and Faris are massive siscons who can't stop arguing that the other one is cuter. Krile's role as the odd girl out is established.

Part3: /g/558642/3d07e1321f/
In which Lenna turns Bartz into a child and then proceeds to traumatize him. Faris objects, right up until she bangs him too.