Is it substantial enough as a first name? Does it fit a variety of personalities? Does it seem modern, vintage, timeless?
[name_f]Belle[/name_f] can definitely stand alone! Itās very pretty, and my first association is Beauty and the Beast, which is a huge plus in my book! I would say it fits a variety of personalities, but I definitely picture a more girly girl. I feel like [name_f]Belle[/name_f] borders classic and vintage.
[name_f]Belle[/name_f] is gorgeous! I have it on my own list as a mn (a bit hesitant to move it to the fn spot because itās so tied to Beauty & the Beast). But I definitely think it stands well on its own. Simple, elegant, strong. I would say it feels pretty timeless, though not in the same way as classics like [name_f]Elizabeth[/name_f] or [name_f]Rose[/name_f]. Modern enough, certainly, but not likely to be trendy. Itās undeniably feminine, but tailored, which makes it feel wearable on any age and any personality. And because of all that, it seems like a fairly perfect choice, imo. 
I agree with the other posters! [name_f]Belle[/name_f] is such a beautiful name. Itās currently on my list, because I love it so much, even though I personally canāt use it (which makes me so sad!)
To me, [name_f]Belle[/name_f] makes me think of someone smart, spunky, strong, kind hearted, and brave. Definitely wearable on different personalities!
I also think the name is timeless, perhaps with a slight modern feel, but definitely not too trendy.
Also, I know the main association is Beauty and the Beast, but lately Iāve been watching a lot of the show Once Upon a Time. Because of that, I view the name [name_f]Belle[/name_f] as less of a āgirly girlā name, and more of a badass one! Although, I think the original [name_f]Belle[/name_f] is just as great, and a wonderful association to have!
[name_f]Belle[/name_f] is both prim and bookish, cheeky and sweet, elegant and fun! I think itās substantial enough:)
Itās adorable and I love it with your sonās names. I was listening to an audiobook today that was read by a little girl named [name_f]Belle[/name_f].
[name_f]Belle[/name_f] is so pretty and cute! Definitely works on its own, as everyone is familiar with it because of Beauty and the Beastā¦
Itās an adorable vintage name, I really donāt think you need to worry about it not being enough on itās own. 

I must sadly disagree with many of the other posters here. To me, [name_f]Belle[/name_f] loses its charm because of these VERY strong associations. I realize some are with [name_f]Bella[/name_f] and not just strictly [name_f]Belle[/name_f], but still, they unfortunately take over the name in my mind, so I donāt see [name_f]Belle[/name_f] as timeless, classic, or vintage- but rather cheap, hokey, and silly. ([name_m]Don[/name_m]āt hate me too much!)
- Disney [name_f]Princess[/name_f] from Beauty and the Beast
- Trendy as a nickname for many of the [name_f]Isabella[/name_f]/Isabelle/Isobels running around now
- Bella is the most popular female dog name (though my hubby LOVES doggy names for people)
- Bella [name_f]Swan[/name_f] from [name_u]Twilight[/name_u]
I love belle! Strongly considered it for my first daughter but my friend used the name [name_f]Isabelle[/name_f] and i thought it was too close! If you think itās too short you can name her [name_f]Isabelle[/name_f], [name_f]Annabelle[/name_f], [name_f]Arabelle[/name_f], or something like that!
Belle certainly can stand one, though honestly feels kind of plain to me as a full name. I think maybe because the meaning is simply āprettyā and there isnāt really anything to it beyond that.
Thereās nothing wrong with that though. [name_m]Plenty[/name_m] of good names have generic meanings.
There are alot of -belle/bel names out there if it doesnāt feel weight enough on itās own to you, and [name_f]Belle[/name_f] or [name_f]Bella[/name_f] is generally considered am intuitive nickname for most of them. As far as longer forms go, [name_f]Mirabel[/name_f]/Mirabelle is one of my favorites.