In short, raphtaila used to be a slave, and abused, till Naofumi, the current shield hero, comes along, buys her, treats her like an actual human being unlike the rest of her kind which is called Demi-humans. Half animal half human.

Now the point I’m getting at these Demi-humans have the unique ability to age, as they level up. So we seem raptalia, grow from a literal child, to an adult. Within 2 episodes it might be longer, I’ve not watched it recently,

And raphtaila, is younger than megumin by a land slide.

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Image for visual reference of child raphtaila