Caitlin Snow (Danielle Panabaker) is receiving the short end of the stick in The Flash season 6, as the character has effectively disappeared ever since she and Killer Frost agreed to share Caitlin's body. This decision has allowed Frost to grow and become more integral to the show, but at the expense of a character who's been a part of Team Flash since season 1.
Caitlin has been juggling her dual identities since the "Flashpoint" storyline in season 3. At the beginning, Caitlin's biggest concern was that her evil meta-human side, Killer Frost, would win out and hurt innocent people. Caitlin went through a lot to keep Frost at bay, but over time, it was discovered that Frost had some redeemable qualities. Later, Caitlin developed a bond with Frost and began transforming into her when situations called for her ice powers. A device allowed Caitlin and Frost to communicate telepathically, which strengthened their ability to coexist.
In The Flash season 6 premiere, Caitlin and Frost reached an agreement. It was decided that Caitlin would start letting Frost take control of their body more often so that Frost can enjoy life. Ever since, the Frost personality has dominated Danielle Panabaker's screen time on The Flash, with Caitlin only emerging on rare occasions. In The Flash season 6, episode 7, "The Last Temptation of Barry Allen, Pt. 1", Frost has what appears to be a panic attack when Cisco (Carlos Valdez) needs her help to save Barry (Grant Gustin). Following the incident, it's agreed that Caitlin will now handle the medical emergencies. This underscores the idea that The Flash hasn't been able to find a balance between the "Caitlin" and "Killer Frost" personalities.
Throughout The Flash season 6, Frost has essentially replaced Caitlin in every capacity, as indicated by the fact that, in this episode, she was doing what should have been Caitlin's job. Caitlin is one of the show's most important characters, yet she hasn't even had an on-screen reaction to the news of Barry's impending death, which is the driving narrative force in season 6, at least leading up to the Crisis on Infinite Earths crossover.
The decision in The Flash's latest episode is the first sign that the series may do a better job of creating a balance between the two personalities going forward, but whether or not this happens remains to be seen. It's possible that Caitlin's connection to Ramsey (Sendil Ramamurthy) will come into play soon. Either way, she has been largely ignored in the first half of season 6, but there's a chance that the character may at least factor more heavily into Crisis on Infinite Earths, especially since one of her closest friends is gearing up to make the ultimate sacrifice.