What does little blue pill do exactly?

[Uncle Jon]

It is a check mark in the adventure mode when doing ITOPOD?

[Fel]

It's a consumable item you can get from the AP shop (and the spin I think but the odds are very low).

When you have some and activate that checkbox, each kill consumes 1 pill and doubles the PP progress gained from it.

[󠀡󠀡]

Little blue pill make your pp go up.

[Elenoe]

and as consumable, blue heart bonus applies. With that one the bonus to PP is x2.,2 (instead of normal x2)