To "Win Isobelle Encarotia's Heart" and earn the "Heart of Isobelle" title in Black Desert Online, you must confess to Isobelle Encarotia at the Valencia Castle Inn with the required Amity and Energy, and have the highest Amity ratio among other confessors when the game's system randomly selects the winner after maintenance. This is a competitive dating system where you compete with other players for the NPC's favor, requiring significant Amity investment to have a chance.
Step-by-Step Guide:

1. Obtain the "Glamorous" Title:
First, you must earn the "Glamorous" title by equipping an Epic or Mystical outfit. This is a prerequisite for interacting with Isobelle

2. Increase Amity with Isobelle Encarotia:

Find Isobelle Encarotia in the Valencia Castle Inn. Gift them STAR DIAMOND
Engage in conversations and interactions with her to build Amity (relationship points).

3. Confess to Isobelle:

Once you have at least 1000 Amity and 5 Energy, you can initiate a confession with Isobelle.

4. Await the Confession Result:

After the confession, Isobelle's response will be delivered via mail after the next in-game maintenance.

The outcome depends on who among all the confessors has the highest total Amity with Isobelle when the system makes its selection.

5. Win the Relationship:

If your Amity is high enough to win, Isobelle will accept your feelings, and you will receive the "Heart of Isobelle" title.

Your Amity will be reduced by 10% as a consequence of forming the relationship.