I'll be adding more feats to the other characters shortly, but Eliza, Marie, and Double should be better adjusted at this point.
Alright, most of the Striking Strength/Attack Potency scale from the Skullgirl and the Skull Heart itself. Marie is one of the weaker Skullgirls that has been created, yet even in her weaker state and constant resistance of the Skull Heart, it still gives her the power to casually lift massive skyscrapers, tank city-busting weapons, and level an entire city with ease.
Now, as for the characters. Each character has their own storyline, and all of the them, with the exception of Double and Fukua, challenge and defeat the Skullgirl at the end. However, not every character destroys the Skull Heart at the end of their story. You see, when the host of the Skull Heart is destroyed, the heart retains the amount of power the bearer had before their destruction. This would mean that even without Marie the Skull Heart would still have at least City Level durability.
Peacock, Ms. Fortune, Painwheel, Squigly, Big Band, and Eliza all destroy the Skull Heart at the end of their respective stories. This would mean that, given the Skull Heart's durability, they would have to have attacks that are either City Level in Attack Potency (Peacock's Argus Agony, Ms. Fortune's Fifth of Dismember, Squigly's Inferno of Leviathan, and Eliza's Khepri Sun) or PJ in Striking Strength (Painwheel's Hatred Install, and Big Band's Satchmo Death Blow).
Then you look at forms like Titan Marie and Blood Deity Eliza. Titan Marie is created when Marie absorbs the entirety of the catacombs beneath the Grand Cathedral. Marie held a Skyscraper aloft AND fought using only 70% of the remains buried within the catacombs, so when she absorbs the remaining 30% her power skyrockets. Blood Deity Eliza on the other hand is created when Eliza absorbs all of the blood she harvested over her lifetime, including the blood from Gehenna. With all of the blood she utilizes she becomes equal to Titan Marie in power.
I hope this brings a little clarity. Like I said, the other profiles will be updated soon. Sorry it took so long to add feats though.