Horohoro is an Ainu from Hokkaido (though the English anime relocates him to "the Northern Woods"). His main drive in the Shaman Fight is saving the Koro Pokkuru, tiny spirit people, which ties into his plan to cultivate a massive Butterbur field. His guardian ghost Kororo has an unrequited crush on Manta Oyamada—a detail fans find amusing.
In the manga, he becomes the third strongest of the Five Elemental Warriors and gains the Spirit of Rain. His furyoku hits 120,000, which is actually higher than both Yoh (108,000) and Tao Ren (100,001)—a funny quirk of the power scaling that makes him technically stronger than characters far more central to the story.
Personality-wise, Horohoro comes across as genuinely good-natured and upbeat, though he'll get serious fast when things matter. He and Tao Ren bicker constantly and seem to clash at every turn, but the manga reveals he cares deeply; he once told You Asakura that if he couldn't save Ren as his friend, he'd never forgive himself. He's also pretty guarded about his past and won't open up about it, even to people close to him. On the competitive side, he's intense enough to literally freeze and shatter Ryuu's arms during a fight at Funbari Onsen.
His name is a running gag—it sounds like "boroboro" in Katakana, which means "crap," so characters constantly mispronounce or misspell it for laughs. His real name is actually Usui Horokeu.
Content compiled by AnimeList.moe from publicly available sources.