How does sentry duty open up in Pokemon mystery dungeon explorers of sky?

Sentry duty will open up after you complete mission 2: Sky Tower: First Ascension with the following Pokemon as your partner:

- Pikachu

- Skitty

- Vulpix

- Chikorita

- Cyndaquil

- Totodile

- Treecko

- Torchic

- Mudkip

- Squirtle

- Bulbasaur

- Charmander