Which princess is the main damsel in distress in the Paper Mario series?

The main damsel in distress in the Paper Mario series is Princess Peach. Across the series, Peach is frequently captured or placed in danger by a villain, giving Mario the central rescue mission.

Peach first appeared as Princess Toadstool in the 1985 game Super Mario Bros. The name “Princess Peach” became the standard English name later, while “Princess Toadstool” remained familiar from earlier games and older translations. They refer to the same Mushroom Kingdom ruler, but Princess Peach is the canonical modern answer.

In the original Paper Mario, Peach invites Mario and Luigi to a party at her castle before Bowser attacks, lifts the castle into the sky, and kidnaps her. Mario spends the adventure rescuing seven Star Spirits so he can overcome Bowser and save her. Peach is not entirely passive: players control her during interludes in which she investigates the castle and secretly helps Mario. Later Mario games increasingly portray Peach as a capable leader and playable hero, but the rescue-princess role remains a defining part of the series' early stories.