What purpose does the blue heart serve in Kronii's ribbon?
She gives it to (You) when she falls in love, literally giving you her heart.
If you hurt or make her sad the heart starts getting cracks in it so you have to live with the knowledge of knowing you hurt her forever. If the relationship ends it breaks and you have to live with knowing you literally broke her heart.
If I broke that off would she die?
Nah, it will mend over very, very long periods of time but leave a very visible discolored scar when it is mended. As much as she eventually wants to forget (You) she will always have that reminder whenever she looks at it.
When she gives it to you, if you inspect it closely you can see various, nearly imperceptible scratches in it already from her being bullied that lead to her current trust issues.
Bullying Kronii back in childhood!
Growing up without realizing how much you traumatized her!
Meeting with Kronii again when you're adults!
Kronii tries to make you fall in love with her in revenge!
Kronii ends up digging in too deep!
You sincerely apologize after realizing Kronii was the weird boy you bullied back!
Trying to break off your relationship with Kronii out of guilt!
Kronii falling victim to Stockholm syndrome and begging you to stay with her!
Having an unhealthy codependent relationship with your menhera gf Kronii!