Pero can be used for a few things:

1. Buying N gacha tickets. There is typically an N slayer card in the gacha during events but no R or SR.

2. Buying N wilds. This is usually not that great and many consider it more useful to get N gacha tickets and spin those instead.

3. Feeding the damn squirrel costs a WHOLE 1k pero!

4. Buying event items or bundles. N gacha bundles currently come with one pink key during the current event. During EG events you can buy shoo-shoo elixirs and smexy drinks.

Most of the time pero is rather useless though, i have 6,5M sitting around.