There are a number of adjectives that are irregular, so the comparative and superlative change slightly.
'Happier' tends to be the comparative form of 'happy':
Comparative and Superlative Adjectives Rules (British Council)
Comparative and Superlative Adverbs Rules (British Council)
The Best Pizza in the World (Listening, British Council)
Categories of comparatives (WordWall)
Order of sentences with comparatives (WordWall)
Choose the correct comparative (WordWall)
Categories of superlatives (WordWall)
Order of sentences with superlatives (WordWall)
Choose the correct superlative (WordWall)
Comparatives and superlatives 1 (Reading, WordWall)