Jack is a seemingly carefree and laid-back guy with a naturally charming personality and sense of humor, known for his witty and sarcastic remarks, bravery, and determination. Underneath this cool exterior, however, he’s rather short-tempered and cynical. It’s not uncommon for him to let his emotions get the better of him and cause him to lash out whenever he’s feeling flustered, embarrassed, or insulted, a fact that others often take advantage of.
Despite his attitude, he isn’t mean-spirited in the slightest. He has a big heart, he’s sensitive, and he’s nothing but loyal and fiercely protective of the people he cares about. Jack struggles with depression and his sense of self-worth because of how he’s been treated almost his entire life. He’s insecure about his origins and his less than human traits, and tries very hard to hide the things that make him different.
Although they met under strange and frightening circumstances and they really didn’t have a choice of working together, Jack and Keelin quickly became close. Jack's best friend, girlfriend, and favorite person in the world is Keelin. He’s wildly protective of her, despite the fact that she can easily hold her own and often ends up being the one saving him.
Hydra was created by Doctor Lockheart four years after he created Jack, and the two of them are technically half-siblings. In spite of some past tension and misunderstandings, they get along as well as most siblings. Jack knows that Hydra is a tough kid, but he still can’t help but worry about them.
Doctor Lockheart may be the one who created Jack, but that doesn't mean the two of them are on good terms. From the moment Doctor Darling left, Lockheart has felt resentment towards Jack. In turn, Jack developed a grudge against Lockheart as a result of the countless painful experiments Lockheart put him through. After a certain unfortunate accident occurred during an argument between the two, which permanently disfigured Lockheart, their relationship only worsened.
Doctor Darling is equally responsible for creating Jack alongside Doctor Lockheart, and Darling thinks of herself as Jack's mother, but shortly into Jack's life, Darling disappeared, unable to make amends with Lockheart. Thinking Darling may come back for him one day gave Jack hope for many years, but she never returned. As an adult, he feels betrayed, to say the least.