Spoiler
On a wild guess, I OEDd 'rath', and it does indeed have an adjectival sense (a variant of 'rathe', meaning quick or eager, and marked with 'rare', 'poet.' and 'dial.' but a word nathless.) And since it is pronounced ra:<theta>, I presume the comparative would be pronounced with a <theta> (as in 'thistle'

to 'rather's <edh> (as in 'this'

.