Trish was the first Lewis - she was Andy Norris' (Holly's bad seed brother) battered wife. After he was locked up, she became involved with Ross in business and sometimes pleasure, and Josh, then a bad boy (basically an Andy who was not going around knocking seven bells out of women), joined her in Springfield. When Pam Long and Gail Kobe came in, they brought in Billy and HB (both of whom had been mentioned for years) and Mindy.
Trish was always much more serious than the rest of the family, and Rebecca Hollen was a very understated, intelligent actress. It's very jarring to see her go from the drab, depressing style of the Marland years to the big hair and big ole fur coat look of Kobe/Long.
She was written out in 1985 (not sure if it was her choice or the show's). She briefly returned for HB's funeral and the last Josh/Reva wedding.