Herman Toothrot (Voiced by Wally Wingert) is an old, ragged castaway hermit residing on Monkey Island from the first up to the fourth installment. He suffers from a partial memory loss and, possibly, severe dementia. Herman accepted his new life on the island and, instead of trying to escape, enjoys his solitude by meditating and often plays mind games with those whom he encounters.
In Secret of Monkey Island, Herman tells Guybrush that about twenty years ago he and his crew mate set sail to find the "secret" of Monkey Island, but got stranded on the island with no means to escape. After his mate died, Herman spent years in vain await of rescue, arguing with Monkey Island's resident tribe of cannibals, and surprisingly, building his own ship (which he couldn't sail all by himself).
In Monkey Island 2: LeChuck's Revenge, Herman resides on Dinky Island in a hammock shack and meditates about life philosophy.
The character had no "live" role in The Curse of Monkey Island other than cameoing as a "dynamo-monkilectric" roller coaster diorama figure at LeChuck's Carnival of the Damned.
Herman returns for the last time in the series in Escape from Monkey Island. Still suffering from severe amnesia, he gradually begins to remember his past with help from Guybrush. His life story, however, turns out different than what was presented in Secret: Herman remembers being in fact Horatio Torquemada Marley, the former governor of the Tri-Island area and Elaine's grandfather (whom the name "Herman Toothrot" borrowed "H.T." initials from), who was en route to uncover the Ultimate Insult before the artifact turns into the evil hands of Ozzie Mandrill. Mandrill, regrettably, had beaten Marley in their race (with LeChuck's help) and was the first to possess the Insult, sending Marley's ship and crew into a whirlpool at the same time. Marley alone survived the wreck, but was marooned on Monkey Island ever since with no memory of these events.
This story was largely criticized by the fan community as being inconsistent with the main storyline of events presented in LeChuck's Revenge. According to it, LeChuck and Horatio were both looking for the treasure of Big Whoop, each of them setting sail to find it about three years ago. LeChuck was the first to reach Big Whoop and stepped in, turning himself undead. Marley tried to stop him, but failed, and disposed of the map to Big Whoop instead, ripping it to four pieces, one of which he kept for himself. When LeChuck confronted the governor in his undead form, he offered him life in exchange for the map piece, but double-crossed him instead and sent Marley's ship into the whirlpool. LeChuck then killed the crew members one by one in search for the other pieces.
Source: Wikipedia, "World of Monkey Island," available under the CC-BY-SA License.