How to know which side of a target actor that our character is approaching?

Development Character & Animation
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2

Using Horizontal Dot product might help. In this example this is your character. When dot is 1 you are directly in front and -1 is directly behind while 0 is side by side.

float DotThreshold = 0.5f;
float DotFromEnemy;
DotFromEnemy = EnemyActor->GetHorizontalDotProductTo(this);
if (DotFromEnemy < (DotThreshold * -1f))
{
     // Behind the EnemyActor
}
else if (DotFromEnemy > DotThreshold)
{
     // In front of EnemyActor
}
else if (FMath::Abs(DotFromEnemy) <= DotThreshold)
{
     // Side of EnemyActor
}