Marius Trevelean doesn't exist, but if he did exist, he would probably look like this guy.

Legend tells he was once chased by an elephant and a herd of hippos (is herd the collective noun? A pack? A sleeve? A something of hippos.) And a handful of crocodiles. 'Handful' is definitely the right word to describe lots of crocodiles.

Marius Trevelean also fell off a waterfall, maybe? It is implausible he would survive, and if he did survive, he would definitely bruise his backside.

Theoretically, Marius Trevelean would be English but would live in America. I would imagine he would also write comedic science fiction stories for incredibly intelligent and funny Middle Graders.

He would not write about hippos. It's too difficult.