Steam – What do Amiibo do in Codename S.T.E.A.M.

As Amiibo level up, their stats will increase. This will lead to Amiibo being statistically more powerful than other players when custom characters are turned off. You can customize how their stats increase by feeding them equipment, but they will increase even if you don't. The total of Amiibo stats (attack + defense + speed) cannot exceed 120.

Amiibo's stats will stop increasing at level 50, although they will continue "learning" and to improve their AI. Amiibo's AI can become considerably better than even a level 9 CPU, and their behaviors will be representative of what they found to be successful, for them as well as their opponents, so far.

According to Wikipedia:

Supported games offer one of two kinds of Amiibo compatibility; the ability to access an Amiibo figure's NFC tag and store data, and read-only recognition. Each Amiibo figurine largely corresponds to a specific game that can access its storage space, though some may have multiple games that can use it. However, each Amiibo figurine can only store data from one compatible game at a time, meaning data must be deleted from the figure in order to use it with a different title.

These 3DS games are:

• Super Smash Bros. for Nintendo 3DS
• Ace Combat: Assault Horizon Legacy Plus
• One Piece: Super Grand Battle! X
• Code Name: S.T.E.A.M.

Here's the compatibility list for each Amiibo.

Important note:
Amiibo figurines can be used directly with the Wii U GamePad and New Nintendo 3DS series, but you need an adapter for the rest of the Nintendo 3DS series to be released in 2015.

To connect an Amiibo to the Wii U, you'll need to place the Amiibo on the rectangular icon on the lower left corner of the gamepad:

For the New 3DS, you only need to place the Amiibo on the touchscreen: