The new game allows you to choose your starting Pokémon partner, but in the demo I had a Chimchar chosen for me. Like the first game, Shadows puts you in charge of rounding up as many Pokemans as possible using a device called the Stylus Styler, which allows you to encircle a Pokémon with some kind of energy field and thus capture it. In gameplay, this means you scribble circles around Pokémon over and over again until their HP elapses. The last game had players circle a certain number of times, but the new one replaces that restriction with an HP bar that decreases as you circle. Eventually, your Styler runs out of power unless you recharge it with a Pokémon's skill.
And that's where the Pokémon you catch come in. Each one you catch has an ability that can be used to assist you with catching more; some recharge your Styler, some slow the wild Pokémon or give some other status effect. They also have effects outside of battle, altering the player's stats and abilities (A Doduo increases your speed, for example).
All of this somehow allows you to solve quests out in the world map, but I'm not sure exactly how that works. My character was asked to assist with the extinguishing of a forest fire, but the demo ended before it became clear how that was going to happen. Probably by catching a water-based Pokémon.
I actually thought Shadows of Almia was pretty fun, mostly because I like action games, and this is a more action-based game than the usual Pokémon. I'm not sure how it would hold up in play sessions longer than a few minutes. Probably exactly as well as the first Ranger.