Nice to meet you, and everyone else.
What do you think is it's biggest weakness/resistance?
I think it's biggest weakness is Steel Types because Umbreon only learns one Ground-Type move, Dig, which is Physical. Steel Types usually have great Defense. So Steel Types are going to be a little problem.
I think it's biggest resistance is Ghost Types because not many Types are resistant to Ghost, and Umbreon pretty much is AWESOME!
I also think Umbreon is cool.