In the Secret of monkey island Guysbrush, (while trying to find Monkey Island) comes across the old captain of his ship's journal, and describes the events of Herman Toothrot and his friend while they search for Monkey Island.
When Guybrush lands on monkey island, with the help of a cannon, strange man introduces himself as Herman Toothrot, however with his head stuck in the sand Guybrush cannot reply, and he leaves.
Herman is a strange old Hermit who refuses to wear pants.
When exploring the island Guybrush comes face to face with him a few times, mostly just spouting useless information. However he does reveal how his ship "The Sea Monkey" made it back to Melee island, he had trained Monkey's to sail it back, however he refused to join them on the ship as he could not stand any amount of time on a ship with a crew of monkey's.
His appearance in the next game is very much the same, however he appears on Dinky island, at first this is not explaines but it is revealed that there is an underground tunnel between Monkey Island and Dinky Island. In this game he is a teacher of Philosophy.
In The Curse of Monkey Island he just has a cameo, on the "Rollercoaster of Death"
Last in Escape from Monkey Island he is revealed as Grandpa Marley, which caused much confusion of the time, and altered much already established canon.
It turns out in this game that once LeChuck and Marley were racing to Big Whoop, and despite being shipwrecked, LeChuck won and got turned into a ghost. Now knowing the danger's of this treasure Marley split the map between his 3 most trusted crew members, Rapp Scallion, Rum Rogers Sr. and Young Lindy.
After these events he travels to Austrailia and takes part in a sailing competition, at this contest he meets Ozzie Mandrill, that night after drinking too much he tells Ozzie of the wealth to be had in the carribean. the next day Ozzie rams Marley into a whirlpool thinking he has killed him but, he winds up with memory loss on Monkey Island and takes the name herman Toothrot from the intials HT etched into an accordian.