Fair is related to Justice. Justice is related to Equality. Equality is the opposite of Difference. Difference is related to Distinguish. 
Mere is related to Slight. Slight is the opposite of Significant. Significant is a synonym of Substantial. 
Fair <--RelatedTo--> Justice <--RelatedTo--> Equality <--Antonym--> Difference <--RelatedTo--> Distinguish 
Mere <--RelatedTo--> Slight <--Antonym--> Significant <--Synonym--> Substantial 