Pippa has a request regarding Blissey, which once helped her out of a difficult situation.

The Request

After you catch Blissey or increase the research level of Blissey's Pokédex entry, check the blackboard in Professor Laventon's office in Galaxy Hall in Jubilife Village to accept this request.

Talk to Pippa

Go to the Alabaster Icelands and talk to Pippa at the Snowfields Camp.

Meet Up with Pippa

Go to the guidance marker and talk to Pippa there.

Battle Abomasnow

This Abomasnow can't be captured, so just defeat it.

Reward

You get 1 Shiny Stone and 2 Max Revive.