SSS: Rise of the Beastmistress

Serena Sterling saves Cassius, who is set to become the Archfiend of the novel she has crossed into. She herself is the fifth princess, a throwaway character who dies in the first chapter, and saving him is her first step away from that fate. She keeps her power hidden after that and builds it up on her own. She tames a tiger, Magnus, and a phoenix, Valerian. The people around her treat her as weak and beneath notice while she works her way up to Tier-10 as a tamer and gains command of three beasts of SSS grade. She turns to darkness as her road to power, breaks the plots laid against her, and beats the people behind them.