The protagonist can initiate the Strength Confidant on May 18th by entering the Velvet Room any time after the mission at Madarame's Palace starts. The requirements for ranking the Strength Confidant are different from a standard Confidant, as they require certain Personas with certain skills.

How do you unlock the strength confidant persona 5 Royal?

In order to unlock the Strength Confidant, you will need to visit the Velvet Room after the in-game date of 5/18. After this, you will asked by Igor to take on a non-compulsory test of strength that will be supervised by Caroline and Justine.

Who is the strength confidant Persona 5?

Persona 5: Fusing Solutions for the Twins' Confidant Cooperation - Twin Wardens Caroline & Justine (Strength)