Jetjaguar

"Jet Jaguar is a human-sized robot created by a Japanese scientist for unknown reasons. Created with a great potential for learning, Jet Jaguar achieved sentience and began to modify himself and his programming. His most impressive modification has been his ability to change his own size-allowing him to grow large enough to dwarf monsters such as Godzilla. Unfortunately, Jet Jaguar cannot maintain his enormous size indefinitely, and must periodically rest to regain his energies. Jet Jaguar is a reserve member of the Global Defense Force-heroically protecting humanity whenever it is threatened by monstrous threats."