With the summer coming to a close, many big TV shows are already gearing up for their fall premieres.

One teen-aimed series just wrapped up its season 3 shoot, and it’s being furiously edited in anticipation of a Sept. 15 premiere. When it comes to the look, feel and themes (love triangles and vampires), the show “Carmilla” isn’t far off from a drama on the CW or Freeform.

However, unlike those cable networks’ top shows, “Carmilla” cranks out 36 (mini) episodes a year for YouTube. It’s also completely funded by an advertiser.

Marketing giant Kimberly-Clark produces the show specifically to promote the young women-aimed feminine hygiene product U by Kotex.

“Carmilla” is based on the vampire-focused novella written by Joseph Sheridan Le Fanu in 1872 and could be mistaken for a scaled-down “True Blood” or “Vampire Diaries.” It’s one of the rare instances of a branded web series that not only resonates with fans, but actually had returned for multiple seasons and appears to be helping improve a marketer’s standing among those hard-to-reach millennial consumers.

The series was commissioned for Kimberly-Clark KMB -0.07% by Shift2, a branded entertainment agency based in Toronto that specializes in helping marketers connect with millennials, and is produced by Smokebomb Entertainment, a production company that works frequently with Shift2.

Since it made its debut in 2014, 72 episodes have been released and most are about five minutes long. Overall, “Carmilla” has generated 43 million views on YouTube, even as the brand has done zero paid advertising to promote it. Jay Gottleib, president of adult and feminine care sector at Kimberly-Clark, said that episodes average about 150,000 unique viewers.

The web show ‘Carmilla’ features occasional product placement from Kotex. Photo: Kimberly-Clark

Producing a full season of “Carmilla” costs $500,000 to $1 million, or roughly a quarter of an average episode of a cable TV show, estimated Jay Bennett, senior vice president of creative and innovation at Smokebomb.

Mr. Gottlieb acknowledged that producing “Carmilla” is still experimental for Kimberly-Clark, and it isn’t easy to prove its value—a common challenge for brands even as they spend more on original content. The company did commission the research firm Fresh Intelligence to survey 10,500 viewers in early 2015, which found that 31% claimed they bought U by Kotex because of the show and 93% knew that the brand was backing the series.

“It’s definitely increased purchase intent among viewers,” he said. “I can’t say this has directly lead to 20,000 new sales, but the best we can say is that it generated positive ROI. Share and volume continues to grow. This little bit is a part of it.”

The brand messaging is subtle. Many episodes don’t feature any mention of Kotex. In some cases, the product or labeling is seen in the background. In one episode, a character throws her ex-girlfriend’s Kotex tampons at her during a breakup argument.

“Fundamentally we didn’t approach this as a branded series,” said Mr. Bennett.

The show is set in modern times despite its 1800s origins and features actors talking to the screen, similar to many YouTube vloggers. Kimberly-Clark’s social media team tries to put out five promotional posts on various outlets for each episode. The show has found a vibrant audience on Tumblr, particular among the LGBT community.

“We set about making a show for a particular audience, which allowed us to have the creative freedom to protect the storytelling from becoming a commercial,” Mr. Bennett said. “After all, you are probably trying to reach the most sophisticated audience in history.”

Write to Mike Shields at mike.shields@wsj.com