Sousei no Aquarion, Comet Lucifer Recommendations
Both are mecha
Both protagonists are young boys who end up entering a world of "protecting the world" by an "accident" of destiny.
Like Comet Lucifer, Aquarion Evol presents two worlds in which one of them desperately wants to be saved.
The romance of both is something almost left in the background, on account of the adventures, however, pleases.