I’m beginning a story and I need a fierce name to go along with my fierce heroine girl detective. Any ideas are welcome!
Depends on what you’re going for…
[name]Charlie[/name]
[name]Teal[/name]
[name]Scarlet[/name]
[name]Vera[/name]
These feel bright & curious to me…like a modern [name]Nancy[/name] [name]Drew[/name]
[name]Calla[/name]
[name]Aria[/name]
[name]Willa[/name]
[name]Jane[/name]
[name]Raina[/name]
These feel more quiet/observant watcher type.
[name]Thais[/name]
[name]Ariadne[/name]
[name]Galina[/name]
[name]Zinnia[/name]
[name]Lake[/name]
Merit
[name]Violet[/name]
[name]Chloe[/name]
Hooray for heroines! If you can, I’d like a bit more description/direction to help me list some more-fitting names for you, but in the meantime I hope these help!
[name]Alexandria[/name] “[name]Alex[/name]” “[name]Lex[/name]”
[name]Andrea[/name] “[name]Andi[/name]”
[name]Arabel[/name] “[name]Ara[/name]”
[name]Auden[/name]
Aven
[name]Avery[/name]
[name]Carys[/name]
[name]Cassandra[/name] “[name]Cas[/name]”
[name]Cora[/name]
[name]Deirdre[/name]
[name]Devin[/name] “[name]Devi[/name]”
[name]Fern[/name]
[name]Gemma[/name]
[name]Jade[/name]
[name]Keira[/name]
Kirean
[name]Kit[/name] (maybe short for [name]Katharine[/name]?)
[name]Lake[/name]
[name]Lark[/name]
[name]Lexi[/name]
[name]Maxine[/name] “[name]Max[/name]”
[name]Miranda[/name]
[name]Paige[/name]
[name]Payne[/name]
[name]Peyton[/name]
[name]Piper[/name]
[name]Quinn[/name]
[name]Raine[/name]
[name]Reagan[/name]
[name]Regan[/name]
Sona
[name]Tatum[/name]
[name]Teagan[/name]
[name]Wren[/name]
[name]Zara[/name]
I’m not sure if this is what you were looking for, but I hope this helped! If I’m completely off-base, please tell me so I can give you some better selections. 
Good luck!
Some spunky, old-fashioned-ish names:
[name]Daisy[/name]
[name]Alice[/name]
[name]Bianca[/name]
[name]Louise[/name]
[name]Maeve[/name]
Some ‘modern’, tough names:
[name]Emma[/name]
[name]Charlotte[/name]
[name]Amelia[/name]
[name]Hannah[/name]
[name]Olivia[/name]
Some random names I think could fit:
[name]Rose[/name]
[name]Rachel[/name]
[name]Tessa[/name]
[name]Eliza[/name]
[name]June[/name]
[name]Jess[/name]
[name]Tab[/name] (Short for [name]Tabitha[/name])
[name]Dex[/name]
[name]Elle[/name]
[name]Hetty[/name]
[name]Penn[/name]
Basically any name that is short and ambiguous in gender
[name]Lexie[/name]
[name]Viv[/name]
[name]Sam[/name]
[name]Tess[/name]
[name]Kat[/name]
[name]Lucy[/name]
Hm. Absolutely agree with above post that unisex names that are short may sound a bit more fierce. It might be interesting to give her more of a frilly name with a spunky nickname, or a formal name that only certain people in her life use. Given she’s your main character, there will be inner conflicts she will face. Names are a great opportunity to demonstrate conflict. Good luck!
A lot of sort of old-fashioned/vintage girl detective names have been suggested, but if vintage isn’t really the style you’re going for, here are some generally fierce names (I’m thinking a long Greek/Latin name that only a sassy girl could pull off, possibly with a matter-of-fact nn):
[name]Cassiopeia[/name] (maybe [name]Cassie[/name] or [name]Cass[/name] for short, you could still probably glean [name]Kit[/name] and [name]Cat[/name] too)
[name]Andromeda[/name] ([name]Andy[/name], [name]Romy[/name])
[name]Alexandria[/name]/[name]Alexandra[/name] ([name]Alex[/name], [name]Sandy[/name], [name]Lex[/name], [name]Lexi[/name])
[name]Delphinia[/name] ([name]Delphi[/name], Phin [sounds like [name]Finn[/name]])
[name]Desdemona[/name] (Dess, [name]Desi[/name]/[name]Dessie[/name], [name]Demi[/name], [name]Mona[/name])
[name]Elisabet[/name]/[name]Elisabeth[/name] (Lis/[name]Liz[/name], [name]Bet[/name]/[name]Beth[/name], [name]El[/name]/[name]Elle[/name]/[name]Ellie[/name])
[name]Iolanthe[/name] ([name]Io[/name], [name]Ianthe[/name], Lan)
[name]Isidora[/name] (Is/Iz, [name]Issie[/name]/[name]Izzie[/name], Dor, [name]Dora[/name])
[name]Calypso[/name] ([name]Cal[/name]/[name]Callie[/name])
[name]Cassandra[/name] ([name]Cass[/name], [name]Cassie[/name], [name]Kit[/name], [name]Cat[/name], [name]Sandy[/name])
[name]Cleopatra[/name] ([name]Cleo[/name])
Lysandra (Liss/[name]Lissie[/name], [name]Sandy[/name])
[name]Adelphia[/name] ([name]Delphi[/name], [name]Addy[/name], Phia/[name]Fia[/name])
[name]Agatha[/name] ([name]Aggie[/name], Gate/[name]Kate[/name])
Aemelia ([name]Mel[/name], [name]Lea[/name], [name]Amy[/name])
[name]Anastasia[/name] ([name]Ana[/name], [name]Natty[/name], [name]Asia[/name], [name]Anny[/name])
Discordia (Diss, [name]Cora[/name], [name]Cory[/name], [name]Di[/name]/D)
[name]Immaculata[/name] ([name]Mac[/name]/[name]Max[/name])
[name]Juno[/name]
[name]Lavinia[/name] ([name]Layla[/name]/[name]Laila[/name], [name]Vine[/name], [name]Vinny[/name])
[name]Magdalena[/name] ([name]Mag[/name], [name]Maggie[/name], [name]Dale[/name], [name]Lena[/name], [name]Dolly[/name])
[name]Xanthia[/name] ([name]Xan[/name])
Clytemnestra ([name]Clyde[/name], Tems/[name]Thames[/name], Ness/[name]Nessie[/name])
I think for a fierce girl detective, I’d go for [name]Alexandra[/name] nn [name]Alex[/name], or [name]Maxine[/name] nn [name]Max[/name].
I’d also go for [name]Blake[/name], a unisex name.
[name]Lydia[/name]…
[name_f]Colleen[/name_f]- for the more [name_f]Nancy[/name_f] [name_u]Drew[/name_u] effect
[name_u]Jaden[/name_u]- for the more strong and tough effect
[name_m]How[/name_m] old is she?