My first ultimate (that I got naturally, without using GameShark to get those digivolution items [which I could've easily bought, but...]) was MegaSeadramon. He evolved from a Coelamon (also got naturally) and I don't even know how. I just kept randomly training it and taking it to fights and one day, it suddenly evolved. I've loved MegaSeadramon to death since that day; he's become my favourite Digimon. It helps that he was mighty awesome and could defeat anything I came across. Sad thing is, he eventually died, but I think I recorded him for the battle arena.
It took me some time to figure out how to properly raise my Digimon. I kept getting Numemons and Vademons before.
Yeah, I think it's if you let it faint twice (which makes only one heart remain). And I think it's also the same with Angemon, but he becomes Devimon instead. And also Greymon, who becomes SkullGreymon. I'm probably wrong about the last two, though.