XJ-8 is the largest of the nine XJ-series robots, resembling a large, bulky, darker-blue version of Jenny with two belly bolts, small pigtails, and a mostly non-expressive face. She is very tall, thickset, and muscular-looking all around.
XJ-8 is a bit of a tomboy. She seems generally emotionless and robotic in personality, and has a deep, monotone, electronic-sounding mechanized voice. She prefers using her brute strength over weapons and strategy.
Due to her robotic nature and lack of a complex personality, XJ-8 finds it difficult to adapt to human cultures. This is shown in Sibling Tsunami, when she mistook a little girl breaking a piñata for a villain, thinking the child was “vandalizing” a colorful paper animal with a stick. When Jenny explained the game to her, XJ-8 used her lasers to blow up the piñata instead of simply hitting it with the stick to break it, burning up all the treats inside and inadvertently ruining the event.
According to Dr. Wakeman (as she explained in Turncoats), XJ-9 was made weaker than XJ-8 because she had made XJ-8 so strong that there was no space left in her for a complex teenage personality, such as sensitivity or sense of humor.
Dr. Wakeman activated XJ-8 again briefly in There's No Place Like Home School. Upon Dr. Wakeman’s request, as an act, XJ-8 dressed in a tuxedo and pretended to be Jenny's (unwanted) "date" to the Fake Prom in this episode. After Jenny had fled the house in response, XJ-8, still wearing the suit, instead acted as a waiter to Dr. Wakeman.
- Sibling Tsunami (First appearance)
- Sister Sledgehammer (She was the last XJ robot to be absorbed by the Cluster-possesed Jenny, but is later freed.)
- Agent 00' Sheldon (Cameo, no lines)
- Escape from Cluster Prime (Brief Cameo)
- There's No Place Like Home School (Brief Appearance)
- Labor Day (Brief Cameo)
- Turncoats (She was mind-controlled to destroy her creator by the evil Dr. Locus but is later freed from the control.)
- XJ-8 is the strongest of the XJ sisters due to her brute strength. However, her reliance on it is what puts her below Jenny in terms of heroism skills.
- She is closest to age with Jenny but still believes she is the older sister because of her size. (Contrary to what Jenny says, XJ-8 is technically older than she is, seeing as she was built before Jenny.)
- She is a taller, muscular, bulky likeness of XJ-9.
- XJ-8 does display some emotions, such as happiness, anger, and sadness, from time to time (mostly shown in her eyes, as well as her mouth, as smiling or frowning).