What does the number under the heart mean?
Solution 1:
These are called Friendship Points. The amount of friendship points you require to level up your friendship level increase per level e.g
- 1 Point for level 1
- 4 additional points to level 2
- 4 additional points to level 3
- 6 additional points to level 4 etc.
It shows this on the Wiki. While you only go up up one level at a time you gain multiple Friendship Points.
Sources:
Friendship Levels
Video Observation