I don't really know what to add without seeming stupid...
Alice calls Reimu a bitch in Imperishable Night-I just realised how snarky she is in all the non-fighting games when talking to Marisa. it makes sense, but I lol'd at their interactions in IN. And she seems bloodthirsty against Eirin... Just going against fan perception as a timid girl, I guess...
Also, I JUST realised that Nue Houjuu is a midboss in TD, making her the first and only Extra Boss to be delegated to midboss level...
Kinda like poor Kogasa, and Stage 1 Yuyuko...
URAMESHIYA!!!!!!!!!!
I believe fans think Alice is timid because her profile states she's a coward. That and she is very mild-mannered in the fighting games a lot of the times, and described as helping humans in such a fashion in Perfect Memento.
However, this is all BECAUSE of the cowardice. Alice IS a psychopath of sorts. As shown in IN, and even in some of the fighting games (she starts to get particularly snarky with Tenshi, for example). In addition, Perfect Memento says she enjoys getting into duels.
If this explanation seems mixed up/contradictory, that would be because Alice's personality is a bit complicated, in large part thanks to that cowardice. ....okay, I'll admit, I'm distracted in real life now too, so apologies if this explanation ends up being a bit incoherent or contradictory.
As pointed out by Suika, she basically avoids the strong youkai while interacting with humans and weaker youkai. So you'll see her acting mild mannered when the threat of strong youkai are around (check out her interactions with the SDM in the fighting games. She even explicitly states she doesn't want to run into the mistress, which likely explains why she's so polite to its denizons. She's more than happy to attack Sakuya when Sakuya is away from the SDM in the fighting games, though) while being snarky when dealing with beings she's familiar with (Reimu, Marisa, and possibly Yukari).
By default, given her interactions with Suika,Tenshi, the three fairies (before she knew they were fairies), and Eirin, she seems to run under the assumption that new people she meets can be taken on. I'm going to assume this is because she's overall confident in her abilities (thus overall willing to tackle new threats without bothering to research/worry about them being too powerful), yet realizes that her ego will be shattered if she's ever defeated while fighting at full power.
Her interaction with the three fairies also shows that it's a bit of a defensive measure. She's hostile to new beings because she thinks they're a threat to her (which might be a bit of a form of cowardice as well, as a being who knows little of fear probably wouldn't get so defensive), but once she finds out the three fairies are just fairies, she suddenly becomes much more friendly to them.
Or... something like that.
Given my current distracted state in real life, I'm not sure if I should be pressing the "Post" button to this or not.
.....eh, oh well, here goes.