The Shielder class is about the pure will to defend everyone indiscriminately. It's the purity that defined early Mash, and that she lost when her Lostbelt experiences taught her that lives are not made equal and that she's capable of choosing that some lives matter to her more than others, not to mention her growing relationship with Lilith throughout the Ordeal eventually taking her to the point she actively wants someone dead. Every time she tries to act like her old self, her Galahad-like, she's hit with a flashback to Tepeu's speech about the flaws of the deinos way (classic Shiki killing a white woman's purity)
But even with Mash's purity ruined, that purity is still built upon the ideals of fairness and impartiality demanded of an arbiter, so both MetaJeanne and Galahad push her toward reclassing to Ruler if she can't be a Shielder anymore. It doesn't happen, of course. Instead she takes pride in her biases and invents Paladin, the class of "Mash Kyrielight will do what she wants without bothering herself with anyone else's arbitrary categories".
Galahad specifically believes that Mash's potential as a wonderful Ruler is being held back by her idolization of him, pushing her to insist in his Shielder class even when it doesn't fit her anymore, so he enables Lilith's awful behavior throughout the whole chapter and randomly takes over as final boss at the end to get Mash to stop respecting him. It doesn't work. Mash still thinks Galahad is the coolest at the end, so he has to give up because she clearly won't listen, plus the Paladin thing renders the whole "problem" moot anyway.