Roseus

Roseus is one of two Ultima Monsters currently in the game. Defeating her unlocks a one-shot CG with Devon.

If castle Warrengard is upgraded enough to have the backyard unlocked, it becomes possible to move her there, but if you haven't seen the CG yet, you'll miss out on it.

Be warned that if you agree to feed her originally, it leads to a game over in which she drains Devon of all his energy. To obtain the CG, you must do battle and win against her.

Location
West of Arlon is the Serenity Bridge. At the north end of it is a weakened wall. Bring some dynamite to open it and traverse through the dungeon that follows. Roseus spawns in the end, but only during the daytime.

Devon Solo (Medium-Hard)
Devon lacks any form of fire attacks, which Roseus is weak to. As such, you'll have to rely on the rest of your kit. While Devon can easily tank several blows without much problem, she is capable of inflicting status ailments, such as poison. Not curing that could quickly spell a recipe for disaster.

With a Team (Easy)
If you're bringing a team, bring Bal'Rana, Gabrielle and Beatrix. The former two have access to high power Fire type attacks, causing you to deal upwards 5-7000 damage per attack. Devon is free to attack or use items to restore the girls MP while Beatrix takes on more of a supportive role, in healing or getting in a few attacks when possible.

When Roseus enters Distress status (around 25% HP left), she'll start spamming Poison so bring plenty of antidotes.

Trivia

 * Her age is unknown.
 * She appears to be a sentient rose.
 * When moved to castle Warrengard, she's promised not to touch any of the workers there, in exchange for Devon feeding her.
 * Despite this, it is currently not possible to do so.