Thanks for moving this, Renrose. 
I love [name_u]Phoenix[/name_u] and [name_m]Griffin[/name_m] and I think they work really well together. However, if I had to choose, I think [name_u]Phoenix[/name_u] actually sounds more feminine than [name_m]Griffin[/name_m]. [name_m]Griffin[/name_m] feels all boy to me.
I think [name_m]Gryffin[/name_m] seems fine for a girl actually! To be specific, a female character but not your daughter!
I personally see [name_u]Phoenix[/name_u] and feminine and [name_m]Griffin[/name_m] as masculine, but I understand why one might think it works on a girl being a mythical creature and all. I don’t feel like my son sharing his name with girls threatens his masculinity at all so if that is what you want to do, go for it, but I would be caught off guard by meeting a little girl named [name_m]Griffin[/name_m].
I don’t think it is bad if it is a character, just not your daughter. And I strongly suggest you spell it [name_m]Griffin[/name_m].