I like her, she has an ego of course but she's genuinely trying to help everyone at all times... But she fails every single time...so yeah, character development when?
According to Kanon and the school nurse, Koyori gets more durable the more she hurts herself. Soon, there will be a time when Koyori just becomes completely invincible.
Yeah there was a chapter where Yori emphasized the usefulness of safe-falling in a really detailed fashion. The nurse scene in this chapter is a callback to that (her saying that Yori hasn't been coming lately).