First complete nobutsuna sensei quest then switch to yasuke and make your way to hideyoshi's residence just south of azuchi castle. When you arrive, get ready to fight 8 akechi soldiers - their attacks are pretty predictable so use the terrain to your advantage, especially those stairways to split them up. They're weak to heavy attacks, by the way.

After dealing with the soldiers, look for lady nene in the courtyard and chat with her. She'll point you toward sokenji temple which sits west of the castle. Once there, find the writing table to trigger an important scene, and that wraps up lady of whispers.

This quest opens up some cool stuff - you'll unlock silent arrows iii plus new story missions like an evening at otsu. The stealth options really open up from here especially when you start the rengakai missions, making infiltration much smoother later on. Tip: the temple area has some tight spaces, so practice those corner takedowns while you're there they'll come in handy.