Kelly Hu starred in the pilot episode of The 100 but didn't return when the show was picked up to series by The CW - here's why. Based on Kass Morgans' book series The 100, the show is set almost a century after a nuclear disaster wiped out most of the population of Earth. Some humans managed to escape and had been living in a space station called The Ark. When it nears capacity, the decision is made to send 100 teenagers and young adults to the surface to see if the planet could once again sustain life.
In the pilot episode of The 100, Hu's name is listed on screen in the opening minutes, along with the other main cast. She played Callie "CeCe" Cartwig, an officer on The Ark and best friend of Dr. Abby Griffin (Paige Turco), the chief medical officer. In Hu's single episode, Abby is condemned to execution by "floating" (that is, purposely sucked into space). Cece agrees to care for her daughter once she's gone, but Abby is saved by the Chancellor at the last moment. Hu wasn't so lucky, as she was dropped from the cast after the pilot for The 100 was made.
It turns out that Hu's Callie was cut due to budget constraints. In 2014, The 100 showrunner Jason Rothenberg explained on Twitter that Hu's character was eliminated after the show was picked up by the network. He pointed out that budgets often change between the making of a pilot and when a show is picked up to series. In that tweet, he also joked that Callie had been floated off camera, thus implying that she was dead. Considering that there wasn't anywhere to go, being killed off is the only thing that makes sense for Hu's Callie.
Rothenberg again addressed why Hu's character didn't return after the pilot in 2015, backing up his original stance. He wrote on Twitter that even though he thought she was great, there just wasn't "enough for her in series." Unfortunately, Hu was a victim of The 100's tightened budget. Being a midseason show that only ran for 13-16 episodes per season, there likely wasn't much Rothenberg could do to keep Callie around, especially in season 1.
Of course, there were so many things that she could've accomplished on The 100. Given her close relationship with members of the Ark's council, she could've had something to do with the decisions that transpired ultimately leading to everyone going back to the ground, not to mention establishing a new order after everyone reached the ground. So while she wasn't integral to the overall plot, nor did she get much screen time in the pilot episode, she could've grown along with everyone else. Then again, in a show like The 100, where there are so many players, it can be difficult to write for everyone. Cutting Hu's Callie loose in order to save money for the series comes as no surprise.