I think something we all got understand about Knife and Suitcase's relationship, romantic or platonic, is that a REALLY big part of Suitcase's arc is her agency.
The overarching theme of Inanimate Insanity II is choosing who you are and making your own decisions, even when it seems there's no way out. A fully-realized Suitcase is one who takes her life and actions into her own "hands" and controls her own life.
And the thing is! Knife wouldn't ever take that from her. In fact, his heart-to-heart with her on the docks was to get her to make her own decisions. He didn't like how Nickel was choosing for her and encouraged her to go against him even when it felt like no one else would side with her.
If Knife ever tried to do anything for Suitcase, make a decision for her, it would break down so much of their bond. Suitcase doesn't need another person to tell her what to do, she needs someone to stand by and trust she can take care of herself.
It's why she throws "I never needed your protection" at Knife in ii17, and why Knife accepts it (for the most part). When faced with certain death, and seeing how determined Suitcase is, he's letting her go. It'll all be over soon, and he's taking in that Suitcase has become what he hoped she would.
AND!! Even then, Knife in ii18 is clearly bruised by that!! He gripes about how no one needs his protection anymore. As much as he knows this is what's best for Suitcase, he's heartbroken that their arc is seemingly over! This is made even worse when he can't even comfort her in an oncoming panic attack. Suitcase pulls herself out of the hallucination. One way or another, Knife was forced to let Suitcase have full agency.
And Knife's sacrifice was an end to his arc because saving Marshmallow wasn't self-serving at all. He got nothing out of it! He most likely won't get a friend out of it like with Suitcase since Marsh most likely won't ever forgive him. He knew that, and he still did it.
The crux of Suitcase and Knife's collective arc is "independent together". They need each other in the way two independent adults need to thrive on their own to support one another. Both their cups must be full before they can pour into the other's.
It's important to know that Knife and Suitcase are their own people separately and together. They're both strong, and together they're unstoppable.
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