The Mysterious Daimugen

Daimugen is probably the most classic mecha (or zord in PR terms) to ever appear since he's the first one with a human disguise (the second was Futaro who could split up to five of himself). He appeared has a seemingly useless fat jerk who had great hidden power within. He was a goofball character, a comic relief but with an important storyline.

The thing is Daimugen unlike Tetraboy has human intelligence like Daimugen. The back story behind him was that he didn't want to fight anymore until he was summoned forward when the Four Deva Kings appeared. With that in mind, he realized if he doesn't fight, the world will perish so he used his diamond to revert to his true self and thus he combined with the other chi beasts to form the Heavy Armor Chi Palace.

During the Daijinryuu crisis, he proved himself being able to outlast that giant monster. He however most of the time chose to appear whenever he was needed and his last battle was the second time against Major Zydos who was reduced into clay dust afterwards. Certainly he will appear again whenever he's needed for mankind but it's not likely for any future crossover.