Al was too competent to pit against the Rangers in cartoons; Tad Stones mentions his great strength and intelligence. He lacked an exploitable flaw, and Sewernose de Bergerac was eventually developed from the remnants of Al's character.
There is no show canon for Sewer Al, since he did not appear in the cartoon. The following information is from his appearances in print.
Sewer Al's appearance varies dramatically. The only constant is that he is a relatively small crocodilian. However, there appears to have been a final model for him, seen in early concept art. He is stooped, moderately fat, wearing a suit, but very disheveled with scraggly hair or a toupee. He is warty and gnarled, with crazed yellow-red eyes. This was reproduced in a coloring book, but his jacket was patched and worn in the book.
Other artists may not have had access to this model when drawing Sewer Al for books. In The Big Cheese Caper, Al is a small, almost cute typical alligator, a chibified version of the crocodile in Kiwi's Big Adventure, and he has donned a train engineer's outfit (apparently for the occasion of guarding a train car.) In The Missing Eggs Caper, he is hiding behind a door, barely visible except for a tusked head and clawed hand - presumably the artist received limited information. The Sewer Al in Disney Adventures is unlike any other - lithe, strong, and almost a lizard, wearing sunglasses, a vest and hat reminiscent of the early 1980s.
Sewer Al is antiheroic at best. He loves reading and collecting books to the exclusion of all else. His ethical system, if he has one, seems to consist of "collect books." The caliber of book does not appear to be relevant. He has accepted books for their classical value, rarity, and sheer size. He is a freelance worker who seems to have a broad skillset and is regularly employed by Fat Cat for specialized tasks. However, he can easily be bribed with more books to betray Fat Cat.
Story and Role
The Missing Eggs Caper
While tracking down bluebird eggs which have been kidnapped, Chip sees a van drop several rodent-sized books. Chip takes the books and dangles them in front of Sewer Al, demanding to know who ordered the kidnapping before he will give them up. Sewer Al reluctantly squeals on Fat Cat.
The Big Cheese Caper
Fat Cat hires Sewer Al, in conductor's garb and reading something on trains, to guard a huge cheese which has been stolen from the train on its way to a competition. Chip bribes him into betraying Fat Cat in exchange for an encyclopedic set of books on cheese, an unwelcome "gift" to Chip from the giant cheese's rightful owner. Sewer Al thinks this is a great deal.
In a coloring book, Gadget suggests that the Rangers visit Sewer Al when a case unusually baffles her: "it's over my head." He demands payment of a hardcover copy of Huckleberry Finn before giving the Rangers a riddle that puts them on the right track.
The magazine, whose CDRR mini-comics follow canon poorly, once had a story about Dale's premonition that a train loaded with cheese will be bombed. Sewer Al pulls off the attack after requesting and receiving a first edition Cooking with Oat Bran. Presumably the book was rare. In an unusual show of loyalty to his employer, Al bats the Rescue Rangers away from the explosives with his tail without trying to deal.
Disney News (now Disney magazine)
In the Rescue Rangers Suitable for Framing feature, Sewer Al's glowing eyes menace the Rangers from inside a storm drain.
The only publicly released fan fiction featuring Sewer Al that is known at this time is the story Wedding Bells Will Chime; Death Knells Shall Clang, by Matt Plotecher.