The word "romance" here is used as a noun.
The word "more" is thuis ab adjective (not an adverb) modifying "romance".
The word "little" here is an adverb modifying "more". Alternatively one could consider "little more" to be a compound adjective, modifying "romance".
But "more" is not an adverb here, nor is it a determiner because it is not:
a word which is used at the beginning of a noun group to indicate, for example, which thing you are referring to or whether you are referring to one thing or several.
The noun phrase here is "a little more romance" and the determiner is "a".