Briar's favorite season is autumn because the weather is nice, and her birthday is in October so there's that :3c And I think autumn represents Briar very well since it symbolizes change and death, which are two important factors in Briar's story, and also the whole aesthetic is very reminiscent of her colors (reds, yellows, oranges).

She personally also likes winter a lot, she likes being all cozy and drinking hot beverages outside. And she doesn't really like summer because it's too hot for her (despite being a fire-type trainer, Briar does not like the cold), and she's also not very fond or spring because she tends to have a lot of allergies