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':
Comparatives 1, listening (Liveworksheets)
Comparing animals (Liveworksheets)
Superlatives 1 (Liveworksheets)
Superlatives 2 (Liveworksheets)
Superlatives 3 (Liveworksheets)
Comparatives and superlatives 1 (Liveworksheets)
Comparatives and superlatives 2 (Liveworksheets)