Jackson Jekyll is a 2010-introduced and all-around character. Due to his mother being of the Jekyll/Hyde lineage, he has a fully monster alter ego by the name of Holt Hyde. His father is a fire elemental, but only Holt holds that heritage. He and Holt do not get along very well because of their different ways, but they do care for each other. Being a human who has grown up among monsters in a world in which the two groups mutually dislike each other, Jackson is a cautious and shy boy who prefers to stick to the rules, but he's not one to be intimidated just because the person before him is a monster. For a while, he wished to be a monster himself to fit in somewhere, but ever since his discovery of Holt he's stepped away from that goal. In addition to being kind and careful, Jackson is very smart. He's a dedicated scientist and one of the best students at Monster High.
Jackson, like his alter-ego Holt, is voiced by Cindy Robinson in the English version of the cartoon.
In the Hebrew version, he's voiced by Dan Kizler, who also voices his alter-ego Holt Hyde.
Jackson's alter ego is Holt Hyde, known as DJ Hyde in the Monster High books; his transformation is triggered by heat in the book and music in the movies or webisodes.
Jackson is friendly and likes to play casketball, though he has a geeky fashion style and is sometimes picked on by other students. He appears to be the only human student at Monster High, and he is confused by his normalcy and feels a bit outcast in a school full of monsters.
In the diaries he once asked Draculaura to bite him, as he was afraid they couldn't really be friends when school started. Draculaura was able to talk him out of it, but she could tell he was still depressed.
In the Monster High book series he is not afraid or uncomfortable about being around monsters and is able to interact with them quite easily. Draculaura noted in her diary that he was unfazed when he met her father, who normally creeps out both human and monsters.
In the cartoon, he seems much more wary around monsters, most likely due to being bullied at school for being a "normie". He even went as far as to say that he "didn't belong", and that no one would want him at Cleo's party in the webisode "Hyde and Shriek".
While the Monster High students are unaware of Jackson's/Holt's dual nature, older monsters can apparently sense his duality. This is evident in Draculaura's 'Basic' diary when she overhears a conversation between her father and Jackson, where Jackson makes a light joke about stakes. Her father later cryptically tells her that Jackson is more than he appears.
Jackson has black hair with blonde ends, blue eyes and a pierced left eyebrow. He has a yin yang symbol between his shoulders. Because of his shy personality, his body language is nervous and he tends to hunch his shoulders.
Jackson, and therefore Holt, has a Yin-Yang tattoo on his back at the base of his neck.
Throughout each storyline, Holt and Jackson have no control over their transformations and change from one into the other under specific circumstances. In the cartoon series, it seems to be music that causes Holt/Jackson's transformation. This is first seen in "Hyde and Shriek." In the books it is heat.
In Holt Hyde's Basic diary, Holt's transformation into Jackson Jekyll normally occurs at sunrise and lasts until sunset. Holt noted that when he was trying to see what the town was like during the day, he blacked out at the moment of sunrise. However, in Jackson Jekyll's 'Between Classes' diary, it's revealed that the trigger for Jackson's transformation has since changed, so that it is now music as in the cartoon. Specifically, "music with a 4/4 time signature played in excess of 90 decibels" triggers the transformation into Holt. However according to the dairy, because of Jackson's "elemental heritage + hyde transformation + teenage boy" status, it is likely to change again at least once more before he hits adulthood.
In the Monster High book series, the change is caused by sweat.
There are times when Jackson's and Holt's memories and abilities leak over to the other. For example, in the diaries Jackson is able to keep up with Deuce Gorgon in a basketball game (to the Gorgon's surprise), while Holt recognizes Draculaura at a club despite the fact it's the "first" time he meets her.
In the Monster High books, his mother is named Sydney. The exact way Jackson's family tree looks is fuzzy at the moment. In Jackson Jekyll's 'Between Classes' diary, it is revealed that it's Jackson's mother from whom the dual nature comes, while his father is revealed to be a fire elemental and implied to be the parent through which Jackson is the cousin to Heath Burns, a fact first mentioned in "Fear Pressure". However, Ghoul Spirit states that Jackson's mother is the sister of Heath's father, making the exact familial relations unclear.
Until he was 16 years of age, Jackson did not know he had a monster half in the form of Holt. He did know both his parents were monsters, but he thought he'd only been given his mother's human side. Neither his parents apparently thought their son's dual nature was something they needed to inform him about.
Especially Jackson's mother is very protective of her little boy, and seeing him get hurt is something that can trigger her transformation into the mode most suited for helping him out. Also according to Holt Hyde's Basic diary, Jackson's mother doesn't keep the same job for long and the family regularly moves to a place closer to her new job.
In the Monster High books, Jackson is regarded by Holt as his brother, as the latter reveals to Melody Carver: "Because DJ likes her. I can't do that to him...or me, or whoever....He's kind of like my brother, I guess." Heath Burns is not related to Jackson in the books, and therefore neither is Harmony Burns.
Jackson has an uncle who owns a farm. It is not stated what side of the family his uncle is on, although it's slightly more likely he is a Jekyll/Hyde because he has no problem only seeing Jackson out.
- Deuce Gorgon - In the diaries Jackson met Deuce at their neighborhood casketball court. After Jackson surprised Deuce by playing well, he cemented the friendship by lending Deuce his sunglasses. In the cartoon, Deuce was the one to convince Heath not to "stonewall" his cousin and more or less has Jackson join his circle.
- Draculaura - In the diaries Jackson and Draculaura live on the same street, and Draculaura brought a pie to his family. He returned the empty pie plate to her house, and impressed Draculaura by not being afraid of her father, and even cracking jokes. He later asked Draculaura to turn him into a vampire, but she refused (then she explained to him why she became a vegetarian). She later brought Jackson to Clawdeen's and her family's annual "Boo-B-Que."
- Frankie Stein - In the webisode "Miss Infearmation", Jackson meets Frankie. Jackson was being bullied by Manny Taur and Frankie jumped in and stopped the bullying. This began a possible attraction storyline arc for Frankie and Jackson. When Jackson and Frankie first shake hands, a cloud formed by an electrical shock from Frankie momentarily forms the shape of a heart. Outside of an impromptu party at Cleo's house, Jackson agonized over going in because he felt he didn't fit in. But the thought of Frankie being there seemed to be the one compelling factor for him, so it seems Frankie has started to have some impact on him. Later, at the party, she also hits it off instantly with Holt Hyde, who seemed quite taken with her as well, going so far as to compliment and flatter her, illustrating that certain things can carry over from one personae to another.
- Clair - In "Ghouls Rule", Jackson meets this gothic girl in the hallways of the normie school, and the two become friends overnight. She helps keep trouble away from Jackson, and even helps the Ghouls of Monster High reunite in friendship with the normies. By the end of the film, it seems as if she wants to be more than friends with him, going as far as to kiss him at the Halloween Party.
Draculaura writes in her Basic diary about her crush on Jackson, and her interest seems to be reciprocated. However, in her School Out diary she writes that the two of them "split up" because Jackson never showed up for any of their dates in the evenings (as by that time he would have changed into Holt Hyde), and could never remember why the next day.
Starting in the webisode "Miss Infearmation", he and Frankie Stein have displayed big feelings for each other. Since then his feelings for her have grown and he seemed disappointed when Frankie tells him they can't go out yet until he works things out with Holt Hyde. However, they are still close friends and he doesn't seem to mind waiting for her.
In the novels he is dating Melody Carver, but in the first novel Cleo kissed him to get back at Melody for bumping into her, causing her to spill her grapes, which (along with the fact of Melody not knowing about DJ Hyde) caused some tension between the two of them.
In "Ghouls Rule", Jackson begins scouting out the normie school, and there meets Clair. The two become fast friends, and by the end of the film, maybe even some thing more, with Clair kissing him at the Halloween Party.
- January 15, 2009: Mattel requests the trademark for Jackson Jekyll.
- Early July, 2010: Jackson Jekyll makes his diary debut in Deuce Gorgon's 'Basic' diary and Draculaura's 'Basic' diary.
- September 01, 2010: Jackson Jekyll makes his Monster High book debut in Monster High.
- Late November, 2010: Jackson Jekyll's debut doll is released as part of the Gloom Beach series.
- May 26, 2011: Jackson Jekyll makes his 2D cartoon debut in "Fear Pressure".
- July 14, 2011: Jackson Jekyll's profile is published on the Monster High website.
- July 15, 2011: Jackson Jekyll's profile art is revealed.
- February 12, 2012: Jackson Jekyll makes his 3D cartoon debut in "Why Do Ghouls Fall in Love?".
- His prototype box seen at the Toy Fair 2012 didn't have his name on the diary and had a different catch phrase which was "To scare is human..."