Does anyone know why in the vitabu Tonks always says "Wotcher"?

It's a greeting. She's saying "What cheer", which has been around for a few centuries now. Basically another way of saying "hello!" :)

Although we spell it 'Wotcha' over here, but I think this is another of those book variations.

i think it means like "What's up?"

It's the British version of "Howzit". At least that's my American take on it. Maybe one of our Brit Marafiki can clarify it further.

It's a slang term for a greeting. Kind of like "Hey," "What's up," etc.