Erica Henderson sat on the worn two-person couch near the entrance of Hub Comics in Somerville when the email that would change her life arrived. She was in the middle of an intense summer back in 2014, traveling back and forth between Somerville and New York, to visit her father, C.J. Henderson who was suffering from cancer before he died on July Fourth.

Henderson was no stranger to comic books thanks to her father, who wrote issues of "Batman," "The Punisher" and, most notably, Neil Gaiman’s "Lady Justice." After graduating from the Rhode Island School of Design in 2008, she dabbled in the comic book industry, freelancing and working on the web comic "The Guilded Age."

“At the time I was doing just enough freelance work to get by, but no more. I just had other stuff on my mind, obviously,” Henderson recalled over coffee in a Somerville café. “But I knew I had to get to working. My dad would have understood, because he lived that same life."

The email she received that July afternoon in 2014 offered her a way to keep doing the work she and her father both loved. An editor of Marvel Comics asked Henderson to create a comic for "The Unbeatable Squirrel Girl."

A cover of "The Unbeatable Squirrel Girl" (Courtesy Marvel Comics)

Co-created by Quincy native and writer Will Murray and the late comic legend Steve Ditko, who drew the character, Squirrel Girl debuted in the back pages of the "Marvel Super-Heroes Winter Special" in the early 1990s. Squirrel Girl is gifted with the proportional strength of a squirrel, a large tail, and an army of squirrel hench-animals. But her real super power, according to Murray, lies in her “unswerving optimism” and ability to relate to those around her.

That’s not to say she isn’t strong. As one of the most powerful characters in the Marvel universe, she’s taken down major baddies including Dr. Doom, Thanos and Galactus.

“I had two objectives in creating Squirrel Girl. First was to evoke the early fun spirit of Marvel Comics that I enjoyed as a reader circa 1962 to '64,” Murray said. “I thought Marvel had turned too dark by the late '80s. Secondly, I consciously wanted to create a breakout character. It appears that I did. It only took about 20 years for her to break out.”

Over its continuing three-year run, "The Unbeatable Squirrel Girl" has sold more than a million issues, won two Eisner Awards and inspired a young generation of diverse fans. In addition to the continuing success of the comic, Squirrel Girl is set to hit the small screen in the recently announced "Marvel Rising: Secret Warriors" animated series, alongside other young superheroines like Spider-Gwen and Ms. Marvel.

An excerpt from "The Unbeatable Squirrel Girl" (Courtesy Marvel Comics)

When Henderson started conceptualizing and sketching ideas for Squirrel Girl in 2014, back on the Hub couch, she hoped to create something completely different from Marvel's other offerings. When Doreen Green (AKA Squirrel Girl) made her debut, the character had an odd circus-inspired look to her, sporting a fur vest and gauntlets, a gray jumpsuit and "KISS"-like makeup. Henderson played a key role in reimagining Squirrel Girl's aesthetic.

Henderson, who credits the work of Dan DeCarlo ("Archie") and Bruce Timm, who developed "Batman: The Animated Series," as inspiration for her art, went with a cartoonish feel for the character, while ensuring Squirrel Girl's body was more realistically proportioned. “I immediately asked if I could redesign the character,” she said. “It was hard because this character hadn’t been in much, and her old costume was just kind of weird."

Henderson set out to give Squirrel Girl a more practical costume: a bathing suit, tights, boots and a bomber jacket. Her headband and acorn earrings were soon added by Marvel. In addition to the new getup, Henderson drew Squirrel Girl as small and sturdy, befitting an athlete who could run stealthily. Embracing a physique almost unknown to the cookie-cutter superheroine design, Henderson fundamentally changed the character.

"The Unbeatable Squirrel Girl" writer Ryan North said it was Henderson's sketches, so full of life and personality, that defined the character. “I wrote the first issue with them up on another monitor, so when I got stuck I could glance at them and ask myself, 'What would this character do?' "

An excerpt from "The Unbeatable Squirrel Girl" (Courtesy Marvel Comics)

Fans flocked to the title, triggering five reprints of the first issue. Launching alongside titles like "Star Wars," "Uncanny" Avengers and "Batman," Squirrel Girl stood out thanks to Henderson’s emotive illustrations and North’s inventive writing.

Brookline resident and Squirrel Girl fan Allison Racicot said the book — and Erica’s art in particular — has inspired her, too. “As a plus-size lady, it was so nice to see that Erica drew Doreen (and every character, for that matter) as a real person, and not just a stereotypical lady superhero," she said. "It meant a lot, and honestly played a big part in giving me the confidence to cosplay Squirrel Girl at New York Comic Con the past couple years."

Ryan North credits Henderson’s eye for realism. The characters she draws look real and wear clothes that real people wear.

“I remember someone on Twitter thanking us for showing Doreen's bra-strap on her shoulder when her shirt shifted a bit because it showed her wearing regular clothes that real people wear, that weren't impossibly idealized,” he said. “She's just a regular person who happens to live in a comic book (and also have squirrel powers).”

Hub Comics manager and lifetime comic book fan Jesse Farrell said the success of Squirrel Girl is multilayered. It’s smart, progressive, serves as a great entry point for kids, he says, adding that it is also incredibly funny.

“It pulls off the Simpsons' trick of making jokes kids will get and jokes that adults will get, while appealing to both,” he said. “Squirrel Girl is an unrelentingly positive and upbeat character in a world that sometimes gets too dark and full of itself. She embodies the best qualities of superheroes, a desire to help, bravery, compassion, and the ability to kick all the butt that's put in front of her.”

Farrell added that Squirrel Girl is one of the top-selling books at Hub Comics.

Lest this kind of earnestness fatigue you, Squirrel Girl also celebrates the utter absurdity of superheroes and affectionately calls attention to their peculiarities without making fun of them. In one episode, Squirrel Girl helps a z-rated villain find a job in New York City, while acknowledging the city's perpetually rising costs.

“Squirrel Girl crossed over in a way I've never seen before and wonder if I'll see again,” Farrell said. “It's not that everyone reads it, it's that so many different people of different ages and walks of life read it. And it meant a lot to me that kids, particularly little girls, loved it and got excited to read it.”

So why did it take 20 years for her to breakout? Most people thought of Squirrel Girl as somewhat of a joke character when she first debuted, including Steve Ditko, Murray said.

“I created her to be a major Marvel superhero," he said. "At the time, I tried to convince my editor that she had tremendous potential, and was capable of reaching audiences Marvel rarely tapped."

A cover of Marvel Comic's "The Unbeatable Squirrel Girl" (Courtesy Marvel Comics)

The new iteration of Squirrel Girl marks a continued push for inclusion on the part of Marvel, who in recent years has added female versions of its favorite characters including Iron Man, Thor and Wolverine in an attempt to court larger audiences.

“Women make up half of all readers so if you know they’re out there and will pay, why not create characters and stories for them,” Henderson said. “That’s on top of the bigger questions of what representation means and why it's important.”

Earlier this year, Henderson announced after more than three dozen issues and a standalone graphic novel of "Squirrel Girl," she would be leaving the book to pursue new opportunities. With issue No. 32, artist Derek Charm picked up illustration duties on "The Unbeatable Squirrel Girl," which is set to continue with North at the helm.

Henderson said she's loved Squirrel Girl and found refuge in reshaping the character after her own father's death, but she's now ready for change.

“I’m very proud to have a part in the character’s development," she said. "It’s amazing when you get to see fans create art, clothing and stories based on something you created.”

Squirrel Girl came into Henderson's life as she struggled with deep loss, offering her an opportunity to infuse new life into a forgotten superhero who has become a new icon of courage.