Hokkaido can be very nice in between late spring and early autum. I travelled there during August and early september and the weather was perfect. It was warm but not hot and humid. Lots of sunshine, generally very good for hiking or other outdoor sports and travelling in general.
Compared to Germany there was more sunshine and the weather didn´t change that rapidly.

Winter is cold with snow and as somebody wrote above, the sea in the north freezes over (its clos to parts of russia already).

Germany does not get this cold by the way.

It's gets pretty cold in new york. I remember one year we got 2 feet of snow and the hudson river had lots of floating ice.

Is it like that or worse in Hokkaido?

There is snow on the ground nearly half the year.

I went during the summer and it was much more comfortable than Tokyo weather-wise.