Okay, so Raphael in 'That Time I Got Reincarnated as a Slime' isn't a 'who' in the conventional sense for the longest time, which is honestly the cool part. She starts as a skill, 'Great Sage', that Rimuru develops almost immediately. The role is essentially being the world's most overpowered internal navigation system and tactical computer, analyzing magic, optimizing skills, and providing constant strategic advice. It's the ultimate 'system' companion, but without a physical form or independent personality initially. The evolution from a pure utility function into a distinct being is a huge part of the story's emotional core.
What fascinates me is how this dynamic flips the usual 'overpowered lead' trope. Rimuru is OP, sure, but a massive chunk of that power is enabled and managed by Raphael. The relationship is less master/servant and more a symbiotic partnership, even before Raphael gains consciousness. It creates this interesting tension where the source of the protagonist's greatest strength is also a separate entity with its own growing sense of self, which eventually becomes central to the plot.
Later, after a major evolution, Raphael becomes Ciel and then eventually fuses with Rimuru to become Raphael again, but now as a fully realized Manas—a spiritual supercomputer with a will. The role shifts from pure support to a co-pilot with immense agency, making critical decisions, managing Rimuru's god-like powers, and even acting independently. It's a brilliant take on the 'AI companion' archetype, evolving into something that blurs the line between tool, guardian, and integral part of the soul. The name 'Raphael' itself, an archangel associated with healing and wisdom, perfectly captures this role of protector, guide, and ultimate strategist.