Fire Rebecca (Rebecca)

Rebecca the Fire Mystic Witch is an highly situational monster, most of the time overrated for her on-the-paper skills especially in Raids where there's no reason to build her instead of Konamiya (Water Garuda), infact she heals a little bit more than him but with a higher cooldown, her first skill debuff doesn't apply to the boss and her second skill cannot apply a defense break with 100% rate because this boss doesn't buff itself.

Despite all the hate for her in other situation we find her suitable as any other natural 3 star, in arena she's good against enemies that applies many debuffs thanks to her third skill that scales based on the the number of debuffs, and if teamed up properly could do AOE defense debuff. She is also useful in dragon B10 if runed violent/nemesis she could do some pretty work as a healer and cleanser.

So if you're planning to build her consider pro e cons and if you have some doubt go fodding her to Megan (Water Mystic Witch).

Fire Mystic With (Rebecca) Suggested Runes

Endgame cleanser - Violent/Nemesis (SPD/HP/ATK) with an eye on speed and accuracy substats
Early Game - Swift/Focus (Spd/Hp%/Hp%)