Hemlock is, technically speaking, not a character in his own right. He is the male alter-ego of the notoriously unstable Laburnum.
He made his first appearance during a Potterverse mission in which Harry Potter, Ron Weasley, and Draco Malfoy were, for reasons which presumably made perfect sense to the author, transported into an alternate universe by the Mary Sue, Princess Rikai, and transformed into Arabian handmaidens with Japanese names. The unfortunate effects of the dimension switch affected the agents as well, switching their sexes for the duration. Laburnum was, at the time, in the middle of a bout of depression and self-hate, and suffering from the desire to become someone else for a while. When she found out that the Official Fanfiction University of Redwall staff member Joseph the Bellmaker-Turned-Mad-Scientist had got hold of the TF-Gun from the webcomic El Goonish Shive, Hemlock fitted the bill nicely. With tutoring from student and self-proclaimed expert boywatcher Suzi Goldenfur, he's rapidly learning to act like a boy as well as look like one.
Drake, unfortunately, has met him without realising who he is. This is unlikely to end well.
In appearance, he strongly resembles Laburnum; on the big side, with curly brown hair or fur, and a sleepy expression. In ferret form, he scruffs up his chinfur to form a goatee, on the basis that he needs all the help he can get to look masculine, and in human form he's often unshaven. In personality, he's also similar to Laburnum, but because he's constantly watching himself to avoid slipups, he's more cautious about his behaviour than she is, and less likely to get into random fights. He's also very shy, mostly because he's terrified of slipping up and being recognised.
By 2010, Laburnum is more open about the existence of Hemlock. He has, by this point, become less shy, as it no longer matters if people know who he is, and more at ease with himself. He and Laburnum insist on talking about each other as if they were separate people, passing for twins to people who don't know them well, and sometimes misleading people into thinking they're either incestuous or telepathic just for laughs.
- "Hemlock" (does not technically appear, but is named)