- For Teachers
A: Is John better behaved than James?
B: John is better.
There's a fine line between grammaticality and incompleteness.
PS There's another sort of 'better', meaning 'recovered from an illness'. This makes a statement like 'I'm better' ambiguous. When I use it, I sometimes change the stress, to emphasize that I'm not yet 'fighting fit': 'I'm better, but...'.