It's in ð IPA but it's root is as an old germanic letter for ð sound, including in English.

As for ð why, I þink ð latin alphabet is maladapted for ð English language as it is currently spoken, and ðat older letters should be written back in.

I mean really I þink ð whole of English spelling should be orþographized, but outside of a chat where I know people are going to be able to read and understand a reformed spelling system, just using ð and þ where I'd speak the according sounds suffices for me :þ