Hidden Rule

[name_f]Anastasia[/name_f] is not valid.

[name_f][/name_f]

I’ll give another hint if there isn’t a valid guess soon.

[name_f][/name_f]

Valid: [name_f]Anya[/name_f]
[name_f][/name_f]Not valid: [name_f]Beatrice[/name_f], [name_f]Amity[/name_f], [name_f]Nancy[/name_f], [name_m]William[/name_m], [name_f]Anneliese[/name_f], [name_m]Beau[/name_m], [name_f]Amy[/name_f], [name_f]Hartleigh[/name_f], [name_m]George[/name_m], [name_m]Anatole[/name_m], [name_f]Hannah[/name_f], [name_f]Anastasia[/name_f]