Since it wasn't answered above; Assist cards are equipped in a separate tab from the action cards. It's in English, so shouldn't be hard to find. You can equip 5 total, and they don't add to your CP cost, so you can use your strongest right away. They give passive buffs, and their main focus is increasing your maximum HP and Lux income. There's a slot for special abilities, but I haven't seen any with them, so I'm not sure if they're present yet.
As far as Assist cards go, I'd recommend searching Wonderland, as I found a bunch of silver Merlin and Cheshire Cat cards that give great bonuses. You might be inclined to go all Cheshire, as his base stats for both are significantly higher then Merlin's. However while his HP keeps going up as you level him, the Lux kind of patters out. Merlin goes the other way, focusing on Lux over HP. So at high levels they're more unique and Merlin becomes a lot more useful. So if you have 5 silver Cheshire Cats it would be good for low levels, but I'd recommend upgrading one or two Merlin's as well to maximize Lux income.
Also, something else it took me a while to figure out. If you mouse over an ingredient when upgrading your keyblade, it will tell you what world it can be found in.