Milim and Rimuru's whole thing is fascinating because it's a constant flip-flop. One minute they're tagging along on some monster-fighting adventure, all genuine excitement and 'let's be besties!' The next, Milim is flexing her Destroyer title and Rimuru is scrambling to keep up without getting pulverized. It feels very true to how childhood friendships can work—intense loyalty mixed with a weird, unspoken competition. I've seen stories that push the rivalry harder, framing them as future rulers destined to clash over their respective domains, but the ones that resonate most with me are where the rivalry is almost playful, a way for them to test each other's growth without real malice. It's less about 'who will win' and more about 'look how far we've both come.'
That underlying respect is the glue. Even when Milim is being chaotic and destructive, she trusts Rimuru to handle it or rein her in. And Rimuru, for all his scheming, never genuinely fears her as an enemy; he sees the lonely kid underneath the power. A lot of fanfics explore that by having them share vulnerabilities neither would show anyone else—Milim talking about the weight of her ancient memories, Rimuru confessing doubts about leading Tempest. It turns the dynamic from a simple power-level comparison into something much richer about mutual support between beings who are fundamentally alone in their respective tiers of existence.