Is the 686 Athena warm enough on its own?

Yes for most resort days. It is insulated, so a base layer plus a light midlayer covers typical winter temperatures. On very cold days add a fleece; on warm spring days you may overheat.

How waterproof is it compared with other ski jackets?

It uses a 10K waterproof and breathable membrane. That handles snowfall and light rain well, but jackets rated 20K resist wetter, longer storms better. It is not a rain shell.

Can I wear it around town, not just skiing?

Yes. The cut is clean and fitted rather than bulky, which is the point of the Athena. The powder skirt and ski pockets are there when you need them and out of the way when you don't.

Does it run true to size?

It is cut fitted rather than relaxed. If you plan to layer a thick fleece underneath, consider sizing up; otherwise take your usual size.

What features does it have for skiing?

It has an attached hood, a powder skirt, wrist gaiters, and pockets for a lift pass and essentials. Those are the core features most resort riders use.

Is it good for backcountry or hiking?

Less so. The insulation and modest venting suit lift-served days better than long climbs. For touring, a lighter shell with pit zips is the better tool.