In the year 845, Marcel apologized to Reiner for inheriting the Armored Titan, for Marcel recommended Reiner to inherit it in order to save Porco. As a Titan appeared, Marcel pushed Reiner out of the way and ended up getting devoured himself.

Why did Porco save Reiner?

Porco and Ymir's deaths are similar in that they're both Jaw Titans who sacrificed themselves to protect Reiner, but the decision to do so was based around the relationship they each had with someone else, with Ymir trying to protect someone and Porco trying to honor someone.

Why did Ymir save Reiner?

Ymir saved Reiner and Bertholt to make sure that Historia would have them on her side when the Beast Titan attacked the walls. Ymir recognized Zeke at a glance–or at least she recognized his Beast Titan form. Without anyone needing to tell her, she knew that it was he who had turned Connie's village into titans.