Thanks for playing and writing the feedback.
I'm happy you liked the characters!
I felt the boss title, healthbar appearing and music starting was enough to indicate the start of a boss battle, but it was also suggested to add cutscene bars to the screen so that might cover the effect you mean.
It's unlikely that I'll update Goldheart, but I'm collecting the feedback I get on Goldheart to make my next game better.
Thanks!