See how often they participate in the script: HERE.
Character Story: A bird with an afro who is owned by Coro. Trill runs a shop near the Forest Temple and will sell Link oil and potions. His store is run on donations, and if you run off without paying, Trill will remember you.
Race Story: Animals appear all throughout Hyrule and are often often helpful in their own ways, either directly or through subtle advice. Link can talk to them when he is in his wolf form.