What do these lines from リンダリンダ by The Blue Hearts mean?

Question

ドブネズミみたいに美しくなりたい, 写真には写らない美しさがあるから。

I understand this as: ”Just like a rat, I want to become beautiful, because you have a beauty that cannot be photographed” (kind of)

First question: How do I know that in the second part, he refers to her (Linda) and is not talking about himself? In the sense of ”I have a beauty that cannot be photographed” (even though that would be a weird thing to say)?

Second question:

Later on, he sings

ドブネズミみたいに誰よりもやさしい, ドブネズミみたいに何よりもあたたかく。

Now, I understand it as

"Just like a rat, you are kinder than anyone, just like a rat, you are warmer than anything"

Now, this can't be right, or can it? Surely he wouldn't call her a rat? But what, then, is this supposed to mean?

Answer 1

How do I know that in the second part, he refers to her (Linda) and is not talking about himself?

You can't. This part refers to neither "her" not "I". 写真には写らない美しさがあるから here means "because rats have beauty that cannot be photographed (even though rats are generally considered as dirty animals)".

Now, this can't be right, or can it?

No it's not right. This part obviously lacks an explicit subject, but nothing suggests this part is talking about "you" or "her." Since the very first line of this song says "I want to be beautiful like a rat", the natural interpretation of this part should be also "Like a rat, I want to be kinder than anyone" and so on.