What's the meaning of "irreplaceable"?

22 May 2020, Paris, France

If it's not Rin, then what's the meaning of "irreplaceable"?

I've been dying for this moment soooooo faaaaar!

What's wrong with their ages? What's wrong with their races? They are destined to be with and to belong to each other. That's all!

Is it that noone can see and touch his heart like her, or is it that he let know one but her to see and touch his heart? Both of the ways, I think. That's why they are for and of each other ♡