HM Slaves are Pokémon used for the sole purpose of using HM moves when other Pokémon can't learn them, or they are unwanted in a Pokémon's moveset. Chuggaaconroy has used several of these in his Let's Plays.
Diglett was caught in Episode 11. She was used for Cut, as well as Dig.
Slave was a Paras caught in Episode 12. He was used for Cut and Flash. He received his name in Episode 17.
Slave's other moves were Scratch and Stun Spore.