Geminia

Geminia is the enemy within Sienna Travers, she's the split personality that was born with her desire to being perfect, when she appears inside of Sienna, her pupils are small and has a mad grin and a golden aura. Supposed to be the "deceased daughter" of Sampson, Eliza.

Her final appearance were in Episode 4 when Sienna suffered a collapse during a mission after gaining Type-4 Ragnarök and when Sienna develops into a more fierce and serious woman.

Personality
She was aggressive, violent and temperamental. Known for being an "assassin" when riding Ragnarök. However, she's also "protective" of her other personality.

Trivia

 * Geminia means Gemini, considering Sienna's Zodiacal sign, and split personality resembling two people.
 * However, Roger is also a Gemini and is the closest mentor to Sienna.