I have noticed through watching speedruns that there is an unlockable deku scrub character called Yves. I'd like to do a Yves run, the problem is I don't know how to unlock them. How do I unlock Yves?
You can unlock Yves by collecting a grand total of twenty Deku Seeds, which you can obtain from defeating Deku Scrubs during the game.
Deku Scrubs are the brown/green leafy fellas that hide underground and shoot seeds at you once you're in their line of sight:
Using a shield to deflect said seeds back at them will kill them - once you do so, a Deku Seed can be collected off of them:
Once you collect 20, you can turn them in at Tingle's cave in the Lost Woods area - the first 10 you turn in will net you a stamina upgrade, the next 10 will unlock Yves for play: