The final episode of Heartcatch Precure had the four main characters fuse into a single character. A giant woman, even! (The final boss somehow turned himself into a planet-sized giant, so the characters pulled out this never-before-seen power to merge into a form that was a match for it.)

The team (Cure Moonlight, Cure Marine, Cure Blossom, Cure Sunshine):



Fused form (not sure it ever got a name):



They never explored the workings of the whole process much, just won the fight, de-fused, and then carried on exactly like before.