Man, this question takes me back to when I first got into the series. Lisa's age is one of those details that feels intentionally ambiguous—like the creators wanted to keep us guessing. She carries herself with this mature, almost world-weary vibe that makes her seem older, but then you catch glimpses of her playful side and wonder. The main character often treats her like a mentor figure, which adds to the ambiguity.
I remember binge-watching the second season and noticing how their dynamic subtly shifts—sometimes she feels like the older sister, other times they seem like peers. The show never outright states it, but there's this one episode where they flash back to their childhoods, and if you freeze-frame, you can spot a calendar that might hint at a 2-3 year gap. Then again, anime timelines are notoriously slippery.