Arlene is an elderly married woman living in Danville.
Arlene was at the Danville park on the day Kermillian's Comet was to fly past the Earth. She was feeding squirrels nuts when Suzy Johnson approached her. She handed Suzy her last nut when the little girl threatened her ("Comet Kermillian").
Arlene worked as a librarian in the bad alternate future caused by Candace. She reminded her that lab coats had to get worn all the time and gave one to Candace ("Phineas and Ferb's Quantum Boogaloo").