Assists (Not to be confused with Assist trophies from the previous games) are secondary characters that act as helpers throughout battle. Unlike the Assist trophies, You can select between 2 helpers or 1 or none at all inspired by Marvel VS Capcom 2.
Upon selecting your characters, you are then asked what special moves you want to give them. They are in the signs of a red button and the arrow depending on what direction that special you chose goes to trigger it. such as -> means that the helper would do a forward special. Specials like Mr Game and Watch's down special, Fox's down special, and Shulk's neutral special are banned as assist specials. In game, you have to press L and R on the NX controller to activate assist. Assists will spawn in front of you , do their special move, down taunt and disappear. You can use assists again for 15 seconds.
Despite them being selected, they are not playable and don't count as "extra lives" for you.
Depending on the helpers and specials you chose, they could range from very helpful (Samus/Megaman/Mr. Game and Watch) to downright bad. (Pikachu,Jigglypuff,Shulk)