There are some figures that are specifically designed to help out a friendly character. Trying to compile a list of all characters that do so. so lets starting listing them.

Harley Quinn


Bruce Wayne


Foggy Nelson


Juston Seyfert


Scarlet Witch


The Chief


Batmite


President Lex Luthor


Mercy


Boosts Combat Values

Ancient One


Weasel


Starbrand



Recorder #451


What are some more that you can think of?

(I started this post thinking you wanted characters by name... and by the time I realized you wanted general, I was already in too deep to back out.)

This Harley gives Joker an ability that lets Harley do abilities... that effectively just support Joker, anyway.
Baby Doll doesn't specify anyone by name, but she's linked to one character you specify. I'd say that qualifies. Honorable mention to Killer Croc basically designed to work with Baby Doll without mentioning her specifically.
Either way, Baxter Stockman is supporting his Mousers.Gotta have the boss.

The TMNT also have a lot of buddies that call them out, especially Raphael (not counting bros helping bros).



They don't always play nice, though.

There's the Yu-Gi-Oh fusions & rituals, too. I left out some obvious pieces like thugs. If you want a list of all supporting characters, that's going to be a very long list.