Ultra-chan!

Ultra-chan! is a magic form of Rin and only appeared in episode 6: Grief and Hope.

Since Rin was the main form, Rin was just too shy to see something, until her brother, Len, who got defeated by Chara in this episode, made her go Yukki Rin sadistic.

How it happened
Rin was fighting Chara, until she found something, TRANSFORMATION. She transformed into Yukki Rin and then sadistically attacked Chara, within the depths of Rin's soul, and something resonated and then came Ultra-chan, the true form of Rin Kagamine.

When Ultra-chan defeated Chara
It happened with her magic spell, and then banished Chara to another dimension. Since Rin didn't give up, Rin ultimately got powerful and destroyed every AU Chara, except for Swap and Swapfell.