Bertha was technically introduced, although unseen on March 17th 1987. She dialed the wrong number and ended up calling Jon: being the desperate man he always is, Jon ended up asking for a date anyway, to which she for some reason agreed.
Bertha's first appearance was the next day, March 18th 1987. Garfield, taking one glance at Bertha, exclaimed "MOMMA MIA!!" before running in terror. Bertha appears in the final panel and asks "What's with your cat", to which Jon sheepishly replies "He's, uh, terrified of daises" (Bertha was wearing ugly pants with daisy prints all over them).
Over the next several strips (Minus March 22nd, one of the rare Sunday strips unrelated to an on-going saga of strips), Jon and Bertha go out on a date, with Garfield happily joining in (mostly for the food). Bertha frequently embarrasses Jon by asking stupid questions like how to get served at a restaurant, before they actually sat down. At the table, when asked how she likes her steak, Bertha jokingly replies "Just run the steer by the table and I'll grab off a hunk!" to-which Bertha and Garfield roar in laughter while an embarrassed Jon face-palms.
Unlike most women who Jon tried to call by phone, Bertha enjoyed going on a date with Jon. Despite this, Jon actually seemed to find the date a complete and total failure.
Bertha was also seen on February 12, 1989 at Jon's party: she has no dialogue in this strip.
In the June 7, 1990 strip, Bertha appeared again, this time at a restaurant in the background next to Liz and Jon´s table eating a turkey leg. She has no dialogue in either of these strips.
Finally, in the January 12 2003 strips, Bertha ran into Jon and Garfield on a supermarket, revealing that she had lost 200 pounds (presumably since their infamous date of 1987). This is to date Bertha's final appearance.
Bertha's first appearance, 1987
Bertha's last appearance, 2003