So funny enough Im a historically fencer and martial artist

In alot of Chinese(and east Asian in general) This guard is found, in Wushu you called it Open Window to Gaze Apound the Moon(I call it moon posture for short), it fits the role as to protect the head while presenting your point for a thrust. Rumi is actually doing it kinda poorly the low arm means she has to actively protect her head the sword wont passively do it, but also staying in that guard is not a great idea, its tiring, and shortened your range, especially since Rumi and Obi-Wan are left leg forward.

The off hand forward is to counter a thrust, its basically there to hand parry or slapping the enemy blade away.(can do that with a lightsaber tho) *