"Death found me when I was a child, so, yeah, I knew what I was getting into."
"I don't think I did, not really. Does that make me naive?"
"It makes you optimistic."
In 2242, when Tilly was nine, she worked out how to make her food synthesizer produce only spumoni ice cream by calculating the exact protein concentration and small particle dispersion of emulsifiers required. (ST: "Runaway")
Tilly described her fourteen-year-old self as a "weird" child, who didn't have a lot of friends and had no one who believed in her, including herself. (DIS: "An Obol for Charon") In 2247, Tilly attended Musk Junior High School for six months, where she befriended May Ahearn, who gave her the nickname "Stilly". (DIS: "New Eden") She transferred schools when her mother got another posting. (DIS: "Point of Light", "An Obol for Charon")
In school, her mother remembered a particularly painful time when she had more trouble climbing a wall than the other children, where Tilly chose to run away rather than confront the challenge. (ST: "Runaway")
Although often socially awkward, Tilly came out of her shell in certain settings, such as the party held on Discovery, even showing aptitude at 20th century beer pong. During the event, Tilly rebuked romantic advances from Lieutenant Rhys. (DIS: "Magic to Make the Sanest Man Go Mad") Paradoxically, Tilly also became a social mentor of sorts to Michael Burnham and facilitated her introduction to future romantic interest, Ash Tyler. (DIS: "Lethe")
In 2257, in recognition for her role in ending the Federation-Klingon War, Tilly was awarded the Starfleet Medal of Honor. She received her officer's commission as an ensign and was placed on the command track. (DIS: "Will You Take My Hand?")
Tilly continued to serve on the Discovery, under the temporary command of Captain Christopher Pike. She participated in the Command Training Program during this time, a "terrible mistake" according to her mother. (DIS: "Brother", "Point of Light", ST: "Runaway")
Later the same year, Tilly volunteered to stay on the Discovery and accompany Burnham when she took the ship forward in time to the 32nd century, with no chance to return. She recorded a farewell message to her mother before she embarked on the journey. (DIS: "Such Sweet Sorrow")
Upon meeting Michael Burnham, who at the time was an inmate being assigned to share quarters with Tilly temporarily, Tilly was delighted at the thought of having a roommate, stating "A roommate is like an automatic, built-in friend." While Tilly's offer of friendship was initially rebuked by a cold Burnham, and returned later in engineering, Tilly soon found herself admiring Burnham's courage. While still lacking confidence, Tilly shared her closely held dream with Burnham: that she would be a starship captain some day. Burnham, admiring Tilly's optimism, took her under her wing – often engaging in physical and dietary training together. (DIS: "Context Is for Kings", "Lethe")
Tilly met the real May Ahearn when she briefly attended Musk Junior High School in 2247. They frequently ate lunch together. Tilly never spoke to May again after her mother was transferred to another posting.
After being injured during an attempt to remove a piece of the dark matter asteroid, in 2257, Tilly started seeing a hallucination of Ahearn. After learning the real Ahearn had died in 2252, Tilly discovered the hallucination she was seeing was in fact a member of a species native to the mycelial network. (DIS: "New Eden", "An Obol for Charon")
Tilly had read everything there was available on Captain Philippa Georgiou, and was exceptionally excited at the prospect of meeting her after her presumed death. (DIS: "Context Is for Kings", "Will You Take My Hand?")
"Yeah, when the people who are supposed to care about you don't listen to you, it's frightening and it's lonely, and it makes you feel like you're crazy or like you're, like, not even there. I get that."
Tilly is the focus of the novel The Way to the Stars. Her grandmother on her mother's side is revealed to be named Adèle, while Tilly's father is named Iain and serves as a lieutenant specializing in xenoarcheology on the science vessel Dorothy Garrod since a year before Tilly's sixteenth birthday. Tilly's mention in the episode "Project Daedalus" of having run away from home at age 16 and engaging in computer-hacking was very likely a direct reference to the events of this novel, which was released prior to the TV episode.
Tilly appears in the "Age of Discovery" expansion to Star Trek Online, voiced by Mary Wiseman. In the tutorial for Discovery-era player characters, set shortly after the Battle of the Binary Stars, Tilly is a third-year cadet at the Academy, whose engineering aptitude allowed her to be chosen as chief engineer for the cadet cruise aboard a Malachowski-class vessel. When the player character, a fourth-year cadet, unexpectedly becomes the captain, Tilly remains as chief engineer until the ship returns to Earth after rescuing the miners of Corvan II, after which she parts ways with the player just before her eventual assignment to the Discovery. Tilly was also the voice for level-up notifications for Discovery-era Starfleet characters, before being replaced by Gabriel Lorca in May 2019.