Here's why Arden Cho didn't return as Kira for Teen Wolf's final season. The original Teen Wolf movie is a 1985 comedy inspired by 1950's b-movie I Was A Teenage Werewolf. This was notably the first horror movie to feature a teenage monster, but Teen Wolf would take a decidedly lighthearted approach to the concept. Michael J. Fox (Designated Survivor) played the titular character, a teen who discovers his werewolf heritage. The movie is a charmingly dated affair, with Fox on top comic form.
Fox opted not to return for Teen Wolf Too in 1987 since he couldn't stand the idea of doing the makeup again, so Jason Bateman (Arrested Development) made his film debut as the character's cousin. Whereas the original was a solid hit the sequel received terrible reviews and underperformed, and plans for a third movie starring Alyssa Milano were scrapped. To the surprise of many, MTV's Teen Wolf adaptation did a good job updating the concept, whilst balancing the horror and teen drama aspects.
The new Teen Wolf was inspired by Buffy The Vampire Slayer, which also a great genre hybrid. Teen Wolf also provided actors like Dylan O'Brien (The Maze Runner) and Tyler Hoechlin with star-making parts. The series featured a number of great supporting characters, including Arden Cho's Kira Yukimura. Kira arrived during the show's third season and while she was initially supposed to be a guest character, she eventually became a series regular.
Kira is revealed to be a kitsune, which is a supernatural spirit that grants power to its wielder. Cho became something of a fan favorite but she appeared to leave the show during Teen Wolf season 5, with Kira staying behind with the Skinwalkers to learn more about her powers, but it heavily implied she would return. Sadly, this turned out not to be the case, with Arden Cho posting a video confirming she wouldn't be returning for season 6 and Kira's storyline was done.
Arden Cho expressed regret Kira wasn't given more of an epic ending on Teen Wolf and that the show only really touched on the Skinwalkers lore. Cho also stated the size of the show's ensemble is one reason Kira didn't return, with a focus being put on other characters instead. It's a pity Kira didn't return for Teen Wolf's final season since her exit felt a little abrupt. That said if she had been brought back and then sidelined in favor of other characters, that would have also been a disappointment.