So I'm a Spider, So What? - Kumo Desu ga, Nani ka?

[DarthEnderX]

The "Girl in White" doesn't act differently from what we've seen. There's that savage smirk when she kills Julius' party off.

She never shuts up internally, but never speaks externally.

I'm sorry, but that argument doesn't fly. Even if she doesn't talk, she still EMOTES like a huge goofball. During her fight with Potimas, she was doing goofy dances every time she gets a solid hit in.

But then, when she's "The White Lady, she's reserved, verbally AND physically. Again for seemingly no reason other than to mislead the audience.

She's actually still the same, deep down, but since she's now a part of an organisation and is aware of the demon lord being able to destroy her, she's toning down her external behavior. Perhaps having humanoid parts, and later a fully humanoid body, actually also makes her behave more like a human in general.

If that's the case, why does having Kumoko in her head make Ariel act like a goofball?

[Kraco]

If that's the case, why does having Kumoko in her head make Ariel act like a goofball?

For who knows how many centuries Ariel has been doing whatever she wants as the demon lord, with nobody in any position to tell her otherwise. Now she has got a broken personality after the alien mind was forcefully fused with hers. It makes sense her inhibitions are lacking. She's like an intoxicated person.