To me, the nickname 'Ace' is one of those tiny details that tells you how Logan thinks of Rory without spelling it out. He doesn't call her 'Rory' with a hundred adjectives — he drops 'Ace' like a little private smile. In the context of 'Gilmore Girls', it reads as a term of endearment that mixes playful teasing with admiration: 'ace' can mean excellent, top-notch, someone who excels, and Logan often uses it when he's genuinely impressed or trying to sound breezy while being sincere.
I noticed he reaches for nicknames when the conversation is intimate or when he's trying to tilt the power balance toward a more relaxed, personal place. Logan's world is full of formal expectations, so nicknaming someone strips that away and makes the moment feel smaller and more his. Sometimes it comes off protective; other times it's flirtatious — like he's handing her a backstage pass into his private language. That ambiguity is part of why it sticks.
Personally, I love that it feels layered: a casual pet name that also signals respect and a kind of complicity. It shows that beneath Logan's bravado there was a way he genuinely saw Rory, which made their scenes oddly tender even when things weren't perfect.