A Female character named Reiden, does it sound like a girl name? She is a tomboy figure who isnt the most fearless person in the world. Also I have looked at a couple name websites and they dont know what the name means. If you do find out tell me plz! 
[name]Read[/name] this:
A rain-slicked pebble sent [name]James[/name]'s foot sliding out from beneath her.
That probably confused you a bit. [name]James[/name], followed by a feminine pronoun? In real life, there are girls called [name]James[/name] but if it’s for a character, it’s always best to have a name that denotes their gender, otherwise you’ll really confuse the reader. (Unless, of course, their masculine name is extremely relevant to the plot.)
Reiden reminds me of either [name]Rayden[/name] ([name]RAY[/name]-[name]DUN[/name]) or [name]Raiden[/name] ([name]RYE[/name]-[name]DEN[/name]), both of which are boy names. So it doesn’t even sound tomboyish to me. [name]Rayden[/name] is a modern invention and [name]Raiden[/name] means ‘thunder god’. [name]Raiden[/name] is the thunder god in Japanese mythology. But if you pronounce [name]Raiden[/name] like [name]Rayden[/name] then it instantly becomes a modern invention.
With tomboyish characters you can always compromise by giving her a name that is unisex but used more for girls ([name]Riley[/name] in the US, [name]Reese[/name] in the US, [name]Emery[/name], [name]Taylor[/name] in the US, [name]Ashley[/name] in the US, [name]Alexis[/name], etc) or you can give her a super feminine name (which, incidentally, the character might hate being called) with a more androgynous nickname that is only used in dialogue. Such as:
[name]Alexandra[/name] nn [name]Alex[/name]
[name]Lavinia[/name] nn [name]Vinnie[/name]
[name]Charlotte[/name] nn [name]Charlie[/name]
[name]Evangeline[/name] nn [name]Van[/name]
[name]Lucretia[/name] / [name]Lucinda[/name] / [name]Lucille[/name] nn [name]Luke[/name] or [name]Luc[/name]
[name]Persephone[/name] nn [name]Percy[/name] or [name]Perry[/name]
[name]Teresa[/name] nn [name]Reese[/name]
[name]Erica[/name] / [name]Frederica[/name] nn [name]Ricky[/name]
[name]Louisa[/name] nn [name]Louie[/name]
[name]Theodora[/name] nn [name]Theo[/name]
[name]Winifred[/name] nn [name]Win[/name] or [name]Freddie[/name]
[name]Francesca[/name] nn [name]Frankie[/name]
Well, you get the picture, lol. [name]Hope[/name] this helps and good luck with your character! 
When I read it, I immediatly think male. While it could be used on a female, I personally wouldn’t use it. [name]One[/name] thing to remember is that people aren’t named after their personalities. [name]Just[/name] because she’s a tomboy doesn’t mean she has to have a boyish name. Knowing the type of people her parents are, and why they chose the name, will tell you a lot more about a character than just giving them a name will. If they seem the type to use unisex or boy-ish names for a girl, then go for it. If they’re more formal or traditional, a more feminine name with a tomboy nickname might fit her better. If it’s some sort of fantasy or futuristic society, the rules of naming get changed around a lot, and you could probably use Reiden without a problem. If it’s set in modern times, you’d need a quirky family to name their daughter that. It’s not impossible, but something to keep in mind.
If you want it to be a girls name then I’d say it is. If you think its right for your character, just go with it.